How to write my own method for ActiveRecord using "act_as_ <something>"?

I am using Ruby on Rails 3 and I am trying to use some kind of custom method for ActiveRecord so that I can do

Account.<my_method_name>

to add, for example, some data to the Account object (but without changing the account data itself).

I heard about the expression acts_as_<something>, and it is used, if I'm not mistaken, when creating plugins, but I did not understand if I needed it. After adding acts_as_<something>, perhaps I need to write somewhere a method similar to the following to “act” on the current ActiveRecord, which is called <my_method_name>:

def self.<my_method_name>
  ...
end

Can someone explain to me how to use this? And, if possible, can you give me a real example of its use?

BTW: lib RAILS_ROOT/app/lib?