Abstract class and Interface

See this CP Article on Abstract Class versus Interface[^]

Some more Links
Interface vs abstract[^]
http://geekswithblogs.net/mahesh/archive/2006/07/05/84120.aspx[^]

Because sometimes there's no need for a class, i.e. you don't need to provide any functionality: you just need to specify a contract (the interface) between you and the folks that will implement the interface.
:)

Both have their use and it is up to you to decide when to use which.

I use Interfaces practically all the time to define the contracts my componets will support or will give support to. Sometimes I myself implement those interfaces, sometimes it is somebody else. When is the case I create one possible, base and generic implementation of the interface. This is good when you want to give some logic ready to your consumers, this make it easier to distribute flexible frameworks and, since you still implement the interfaces there is always the possibility of doing it all from scratch and or replace parts of the pipe.

Interfaces also are a great deal of help when unit testing you componets, you dont need to have all of your dependencies ready when you develop, having the interface you can mock those dependencies (using Mocking Framewors) and test your component without any problems.

The more you become a professional developer, more you will see the differences between abstracs and interfaces. For now, I hope I helped giving you an entry point for you to find out when and where to use them for yourself.