Let's look at the RMagick equivalent of "Hello, world". This program reads an image file named "Cheetah.jpg" and displays it on your monitor.
1. require 'RMagick'
2. include Magick
3.
4. cat = ImageList.new("Cheetah.jpg")
5. cat.display
6. exit
  Line 1 requires the RMagick.rb file, which defines the Magick module. The Magick module contains 3 major classes, ImageList, Image, and Draw. This section - Basic Comcepts - describes the ImageList and Image classes. The Draw class is explained in the Drawing on and adding text to images section, below.
The statement on line 5 creates an imagelist object
  and initializes it by reading the Cheetah.jpg file
  in the current directory. Line 6 sends the display
  method to the object. When you send display to an
  imagelist, it causes all the images in the imagelist to be
  displayed on the default X Window screen. In this case, the
  display method makes a picture of a cheetah appear
  on your monitor.
Type this program in and try running it now. The
  Cheetah.jpg file is in the ex/images
  subdirectory where you installed the RMagick documentation.
The Image and ImageList classes are closely related. An Image object describes one image or one frame in an image with multiple frames. (An animated GIF or a Photoshop image with multiple layers are examples of images with multiple frames.) You can create a image object from an image file such as a GIF, PNG, or JPEG. You can create a image from scratch by specifying its dimensions. You can write an image to disk, display it on a screen, change its size or orientation, convert it to another format, or otherwise modify it using one of over 100 methods.
An ImageList object is a list of images. It contains zero or more images and a scene number. The scene number indicates the current image. The ImageList class includes methods that operate on all the images in the list. Also, with a very few exceptions, any method defined in the Image class can be used as well. Since Image methods always operate on a single image, when an Image method is sent to an imagelist, the ImageList class sends the method to the current image, that is, the image specified by the scene number.
The ImageList class is a subclass of the Array class, so you
  can use most Array methods to change the images in the imagelist.
  For example, you can use the << method to add
  an image to the list.
Going back to the example, let's make one modification.
1. require 'RMagick'
2. include Magick
3.
4. cat = ImageList.new("Cheetah.jpg")
5. smallcat = cat.minify
6. smallcat.display
7. exit
  The difference is the statement on line 5. This statement
  sends the minify method to the imagelist. The
  minify method is an Image method that reduces the
  size of an image to half its original size. Remember, since
  minify is an Image method, the ImageList class sends
  minify to the current (and only) image. The return
  value is a new image, half the size of the original.
Line 6 demonstrates the Image class's display
  method, which displays a single image on the X Window screen.
  Image#display makes a picture of a (in this case,
  small) cheetah appear on your monitor.
Here's how to write the small cheetah to a file in GIF format.
1. require 'RMagick'
2. include Magick
3.
4. cat = ImageList.new("Cheetah.jpg")
5. smallcat = cat.minify
6. smallcat.display
7. smallcat.write("Small-Cheetah.gif")
8. exit
  The statement on line 7 writes the image to a file. Notice
  that the filename extension is gif. When writing
  images, ×Magick uses the filename extension to determine
  what image format to write. In this example, the
  Small-Cheetah.gif file will be in the GIF format.
  Notice how easy it is to covert an image from one format to
  another? (For more details, see Image formats and filenames.)
So why, in the previous example, did I create cat
  as an ImageList object containing just one image, instead of
  creating an Image object? No reason, really. When you only have
  one image to deal with, imagelists and images are pretty much
  interchangeable.
Note: In most cases, an Image method does not
  modify the image to which it is sent. Instead, the method returns
  a new image, suitably modified. For example, the resize method returns a new
  image, sized as specified. The receiver image is unaltered.
  (Following the Ruby convention, when a method alters the receiver
  object, the method name ends with "!". For example, the resize! method resizes
  the receiver in place.)
You've already seen that you can create an imagelist and
  initialize it by specifying the name of an image file as the
  argument to ImageList.new. In fact,
  new can take any number of file name arguments. If
  the file contains a single image, new reads the
  file, creates an image, and adds it to the imagelist. If the file
  is a multi-frame image file, new adds an image for
  each frame or layer in the file. Lastly, new changes
  the scene number to point to the last image in the imagelist. In
  the simple case, new reads a single image from a
  file and sets the scene number to 0.
Underneath the covers, new calls the Image
  class's read method to read the
  image file. The read method is a class method. Given
  one or more file names, read reads the files and
  constructs a Image object for each image in the files. The return
  value is an array of one or more images. Upon return,
  new simply appends the new images to the
  imagelist.
You can also create an image from scratch by calling Image.new. This method takes 2
  or 3 arguments. The first argument is the number of columns in
  the new image (its width). The second argument is the number of
  rows (its height). If present, the 3rd argument is a Fill object. To add a
  "scratch" image to an imagelist, call ImageList#new_image. This
  method calls Image.new, adds the new image to the
  imagelist, and sets the scene number to point to the new image.
  Scratch images are good for drawing on or
  creating images by compositing.
Like many other methods in the Image and ImageList classes,
  Image.new accepts an optional block that can be used
  to set additional optional parameters. If the block is present,
  Image.new creates a parameter
  object and yields to the block in the scope of that object.
  You set the parameters by calling attribute setter methods
  defined in the parameter object's class. For example, you can set
  the background color of a new image to red with the
  background_color= method, as shown here:
require 'RMagick'
include Magick
# Create a 100x100 red image.
f = Image.new(100,100) { self.background_color = "red" }
f.display
exit
  Within the parameter block you must use self so
  that Ruby can knows that this statement is a method call, not an
  assignment to a variable.
Similar to Image.new,
  ImageList#new_image also accepts an optional block,
  passing it on to Image.new.
You can create an image by capturing it from the XWindow
  screen using Image.capture. This method
  can capture the root window, a window identified by name or ID
  number, or perform an interactive capture whereby you designate
  the desired window by clicking it or by drawing a rectangle on
  the screen with your mouse.
Both the Image class and the ImageList class have
  write methods. Both accept a single argument, the
  name of the file to be written. Image#write simply writes
  the image to a file. Like the Image#read method,
  write yields to an optional block that you can use
  to set parameters that control how the image is written.
If an ImageList object contains only one image, then ImageList#write is the same
  as Image#write. However, if the imagelist contains
  multiple images and the file format (determined by the file name
  extension, as I mentioned earlier) supports multi-frame images,
  Image#write will automatically create a multi-frame
  image file.
For example, the following program reads three GIF files and
  then uses ImageList#write to combine all the images
  in those files (remember, each input file can contain multiple
  images) into one animated GIF file.
#! /usr/local/bin/ruby -w
require 'RMagick'
anim = ImageList.new("start.gif", "middle.gif", "finish.gif")
anim.write("animated.gif")
exit
  RMagick defines 3 methods for displaying images and
  imagelists. Both the Image class and the ImageList class have a
  display method. The Image#display method
  displays the image on the default X Window screen. For imagelists
  with just one image, ImageList#display is
  identical to Image#display. However, if the
  imagelist contains multiple images,
  ImageList#display displays each of the images in
  turn. With both methods, right-clicking the display window will
  produce a menu of other options.
The ImageList#animate method
  repeatedly cycles through all the images in an imagelist,
  displaying each one in turn. You can control the speed of the
  animation with the ImageList#delay=
  method.
Once you've created an image or imagelist, what can you do with it? The Image and ImageList classes define over 100 methods for examining and modifying images, both individually and in groups. Remember, unless the ImageList class defines a method with the same name, you can send any method defined in the Image class to an instance of the ImageList class. The ImageList class sends the method to the current image and returns the result.
The methods can be classified into the following broad groups. ImageList method descriptions look like this. Some of the listed methods are not available in some releases of ×Magick. See the method documentation for details.
Image and ImageList objects can be serialized using Ruby's
  Marshal module. Marshaling is supported via
  ×Magick's Binary Large OBject functions
  ImageToBlob (for dumping) and
  BlobToImage (for loading).
The Draw class is the third major class in the Magick module. This class defines two kinds of methods, drawing methods and annotation methods.
×Magick supports a set of 2D drawing commands that are
  very similar to the commands and elements defined by the W3C's
  Scalable
  Vector Graphics (SVG) 1.1 Specification. In RMagick, each
  command (called a primitive) is implemented as a method
  in the Draw class. To draw on an image, simply
Draw class.draw
    method.The primitive methods do not draw anything directly.
  When you call a primitive method, you are simply adding the
  primitive and its arguments to a list of primitives stored in the
  Draw object. To "execute" the primitive list, call
  draw. Drawing the primitives does not destroy them.
  You can draw on another image by calling draw again,
  specifying a different image as the "canvas." Of course you can
  also draw on an image with multiple Draw objects,
  too. The canvas can be any image or imagelist, created by reading
  an image file or from scratch using
  ImageList#new_image or Image.new. (If
  you pass an imagelist object to draw, it draws on
  the current image.)
 
    Here's an illustration of the default drawing coordinate system. The origin is in the top left corner. The x axis extends to the right. The y axis extends downward. The units are pixels. 0° is at 3 o'clock and rotation is clockwise. The units of rotation are usually degrees.1
You can change the default coordinate system by specifying a scaling, rotation, or translation transformation.
(Click the image to see the Ruby program that created it.)
RMagick's primitive methods include methods for drawing points, lines, Bezier curves, shapes such as ellipses and rectangles, and text. Shapes and lines have a fill color and a stroke color. Shapes are filled with the fill color unless the fill opacity is 0. Similarly, shapes are stroked with the stroke color unless the stroke opacity is 0. Text is considered a shape and is stroked and filled. Other rendering properties you can set include the stroke width, antialiasing, stroke patterns, and fill patterns.
As an example, here's the section of the Ruby program that created the circle in the center of the above image.
 1. !# /usr/local/bin/ruby -w
 2. require 'RMagick'
 3.
 4. canvas = Magick::ImageList.new
 5. canvas.new_image(250, 250, Magick::HatchFill.new('white', 'gray90'))
 6.
 7. circle = Magick::Draw.new
 8. circle.stroke('tomato')
 9. circle.fill_opacity(0)
10. circle.stroke_opacity(0.75)
11. circle.stroke_width(6)
12. circle.stroke_linecap('round')
13. circle.stroke_linejoin('round')
14. circle.ellipse(canvas.rows/2,canvas.columns/2, 80, 80, 0, 315)
15. circle.polyline(180,70, 173,78, 190,78, 191,62)
16. circle.draw(canvas)
  The statements on lines 4 and 5 create the drawing canvas with
  a single 250x250 image. The HatchFill object fills
  the image with light-gray lines 10 pixels apart. The statement on
  line 7 creates a Draw object. The method calls on lines 8-15
  construct a list of primitives that are "executed" by the
  draw method call on line 16.
The stroke method sets the stroke color, as seen
  on line 8. Normally, shapes are filled (opacity = 1.0), but the
  call to fill_opacity on line 9 sets the opacity to
  0, so the background will show through the circle. Similarly, the
  stroke lines are normally opaque, but the tomato-colored stroke
  line in this example is made slightly transparent by the call to
  stroke_opacity on line 10. The method calls on lines
  11 through 13 set the stroke width and specify the appearance of
  the line ends and corners.
The ellipse method call on line 14 describes an
  circle in the center of the canvas with a radius of 80 pixels.
  The ellipse occupies 315° of a circle, starting at 0°
  (that is, 3 o'clock). The polyline call on line 15
  adds the arrowhead to the circle. The arguments (always an even
  number) are the x- and y-coordinates of the points the line
  passes through.
Finally, the draw method on line 16 identifies
  the canvas to be drawn on and executes the stored primitives.
The annotate
  method draws text on an image. In its simplest form,
  annotate requires only arguments that describe where
  to draw the text and the text string.
Most of the time, you'll want to specify text properties such
  as the font, its size, font styles such as italic, font weights
  such as bold, the fill and stroke color, etc. The Draw class
  defines attribute
  writers for this purpose. You can set the desired text
  properties by calling the attribute writers before calling
  annotate, or you can call them in an image block
  associated with the annotate call.
The following example shows how to use annotate
  to produce this image.
 1.   #! /usr/local/bin/ruby -w
 2.   require 'RMagick'
 3.
 4.   # Demonstrate the annotate method
 5.
 6.   Text = 'RMagick'
 7.
 8.   granite = Magick::ImageList.new('granite:')
 9.   canvas = Magick::ImageList.new
10.   canvas.new_image(300, 100, Magick::TextureFill.new(granite))
11.
12.   text = Magick::Draw.new
13.   text.font_family = 'helvetica'
14.   text.pointsize = 52
15.   text.gravity = Magick::CenterGravity
16.
17.   text.annotate(canvas, 0,0,2,2, Text) {
18.      self.fill = 'gray83'
19.   }
20.
21.   text.annotate(canvas, 0,0,-1.5,-1.5, Text) {
22.      self.fill = 'gray40'
23.   }
24.
25.   text.annotate(canvas, 0,0,0,0, Text) {
26.      self.fill = 'darkred'
27.   }
28.
29.   canvas.write('rubyname.gif')
30.   exit
  This program uses three calls to annotate to
  produce the "etched" appearance. All three calls have some
  parameters in common but the fill color and location are
  different.
First, the statements in lines 8-10 create the background. See
  Fill classes for
  information about the TextureFill class. The
  "granite:" image format is one of ×Magick's built-in image
  formats. See "Built-in
  image formats" for more information. The statement on line 12
  creates the Draw object that does the annotation. The next 3
  lines set the values of the attributes that are common to all 3
  annotate calls.
The first annotate argument is the image on which
  the text will be drawn. Arguments 2-5, width,
  height, x, and y, describe
  a rectangle about which the text is drawn. This rectangle,
  combined with the value of gravity, define the
  position of the text. When the gravity value is
  CenterGravity the
  values of width and height are
  unused.
The first call to annotate, on lines 17-19, draws
  the text 2 pixels to the right and down from the center. The
  self.fill = 'gray83' statement sets the text color
  to light gray. The second call to annotate, on lines
  21-22, draws dark gray text 1.5 pixels to the left and up from
  the center. The last call, on lines 25-27, draws the text a third
  time, in dark red, exactly in the center of the image.
The next section, "ImageMagick/GraphicsMagick Conventions," describes some conventions that you need to know, such as how ×Magick determines the graphic format of an image file, etc. The ImageMagick (www.imagemagick.org) and GraphicsMagick (www.graphicsmagick.org) web sites (from which much of the information in these pages has been taken) offers a lot of detail about ×Magick. While these web sites don't describe RMagick, you can often use the documentation to learn more about a RMagick method by reading about the Magick API the method calls. (In the Reference section of this document, most of the method descriptions include the name of the Magick API that the method calls.) Check out the example programs. Almost every one of the methods is demonstrated in one of the examples.
Good luck!
1The rotation attributes rx and
    ry in the AffineMatrix class
    use radians instead of degrees.