Interfaces in apex classes?

Hi Manpreet

An interface is like a class in which none of the methods have been implemented—the method signatures are there, but the body of each method is empty. To use an interface, another class must implement it by providing a body for all of the methods contained in the interface.

Interfaces can provide a layer of abstraction to your code. They separate the specific implementation of a method from the declaration for that method. This way you can have different implementations of a method based on your specific application.

For more information you can refer to this link:http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_classes_interfaces.htm

Hope it may helps: