I'm still confused about the meaning of interface, and abstract classes, and don't know what is the difference between them and ordinary class, and in what situations we use interface or abstract classes.

I read many tutorials about this topic but still confused about their meaning and functionality, and i'd be glad if some one explain their meanings and function by examples.

Regards.

Edit: actually i know what they are and what they do, but i want to know why should i use (abstract or interface) classes instead of just ordinary classes, although they can do the same job well.