The most important thing to get, and something that takes a while to get a handle on at first, is that in ruby, everything is an object. Everything. Not just the thing you made a class out of, but everything you use. When you define a variable, that variable points to an object. When you create an array, that array is an object.
More importantly, every object belongs to a class. Read that again. Every object belongs to a class. If you go far up the chain, every object actually inherits from the Object class, which in turn inherits from the BasicObject class. So really, when we say "everything in ruby is an object" we mean, "everything in ruby inherits from the Object class".
This is super easy to test. Just go into irb, and make an object. Any object. I picked an Array, but a string or a fixnum or even a class you made yourself will do. Try typing the name of the object and then class. As so
=> ArrayThen, add .superclass on top of the class method call.
=> ObjectDo it one more time
=> BasicObjectThis is as far down as the chain goes, if you do it one more time, you get "nil", which means there's no super super super class for array.
What does this mean? It means that if you use an Array you can use all the methods contained in both Object and BasicObject. Object contains many useful methods, like for example, class, which we just used. Which brings us to the next point of discussion, method calls. A method call is quite simply when you call an operation on an object (anything in ruby!) by using a period and then the command. What's interesting is what happens when you call a method on an object.
First ruby checks the class of the object in question and scans from top to bottom looking for the method in question. If it finds it, it runs the last instance of it found, if not it goes into whatever class we inherited from, all the way up to BasicObject. If it finds nothing, it returns an error!
Were you to run SampleClass.new.sample_method, you'd simply get an output of "this method is called", as opposed to "this method is not called" because ruby will always take the last definition. Similarly, if you made a class called say "SonClass" that inherited from "SampleClass" and tried to run sample_method on it, it would just grab it from SampleClass, unless that is, you defined sample method in SampleClass...
Its these properties that make ruby so modular, since you can open up Array and edit its internal functions, or even add functions that can be used by anyone calling on an Array...