Rail Mail is a plugin for ruby on rails that gives you a persisted view of any mail your application has sent. It is both a development aid and an administrative helper.

When Railmail is enabled, any mail sent by your application will be trapped and stored in a database table. An interface is enabled within your application where you can view these emails. With railmail there is no need to set up testing email accounts while developing your application; just send out mail to any address and Railmail will capture it. Click Here to know more

include mixes a module into a class or another module

extend is used to include a module in an object(instance).

load will load and execute a ruby program(*.rb)

require loads ruby program as well, but will also load binary ruby extension modules(shared libraries or dynamic link libraries).
In addition, require ensures that a feature is never loaded more than once.