Ruby "Base" classes

It seems commonplace to name classes "Base" in Ruby. I'm not sure why, nor how I feel about it.

Consider, for example, ActiveRecord. ActiveRecord is a module that contains a number of classes such as Observer and Migration, as well as a class called Base. What's the benefit of this, as opposed to having an ActiveRecord class that contains Observer and Migration?

class ActiveRecord

  class Observer
    [...]
  end

  class Migration
    [...]
  end

end

vs

module ActiveRecord

  class Base
    [...]
  end

  class Observer
    [...]
  end

  class Migration
    [...]
  end

end