what's different between interface and abstract class?

[Dejan Lekic]

Allan, think of interfaces as a "contract" betwen a programmer and user of the class, which states that a certain class (which implements the interface) MUST have certain methods declared in the interface.
Thus interface contains only SIGNATURES of methods that the class must implement.

Class, however is, simply said, a template for making objects. It contains members' and methods' definitions.