ActiveRecord::Migration is a mix-in?

I'm going by <http://railsforum.com/viewtopic.php?id=1011> and am curious
about this line:

class AddUserIdToPosts < ActiveRecord::Migration

AddUserIdToPosts is a class, ok. ActiveRecord is a module, ok.
Migration is a class? This is an example of a mix-in?

I don't really understand this line :frowning:

-Thufir