I'm familiar with function definitions and variable declarations being in class definitions:

public class MyClass {
  public int myvar;
  public void doSomething() {

  }
}

But what does it "mean" in Ruby when a method is actually called in the class definition? This happens amply in Rails, for instance:

class User < ActiveRecord::Base
  has_many :posts
end

What exactly does this do (at a lower level than "it adds some methods to the class")? How would I implement such a function (e.g., one that mixes in some additional methods)?