What is the difference between Integer and Fixnum?

UPDATE: As of Ruby 2.4, the Fixnum and Bignum classes are gone, there is only Integer. The exact same optimizations still exist, but they are treated as "proper" compiler optimizations, i.e. behind the scenes, invisible to the programmer.

This is somewhat confusing. Integer is the real class that you should think about. Fixnum is basically a performance optimization that should never have been made visible to the programmer in the first place. (Compare this with flonums in YARV, which are implemented entirely as an optimization inside the VM, and never exposed to the programmer.)

Basically, Fixnums are fast and Bignums are slow(er), and the implementation automatically switches back and forth between them. You never ask for one of those directly, you will just get one or the other, depending on whether your integer fits into the restricted size of a Fixnum or not.

You never "use" Integer. It is an abstract class whose job is to endow its children (Fixnum and Bignum) with methods. Under effectively no circumstances will you ever ask for an object's class and be told that it is an Integer.