Ans:

Abstract Class:
Instance can not be created. Optionally can have one or more abstract methods but not necessary. Can provide body to Classes.

Interface:
What a Class must do, But not how-to.
Bridge for the communication when the caller does not know to whom he is calling.
Describes externally visible behavior of element.
Only Public members which defines the means of the communication with the outer world. Can-be-Used-As Relationship.
Can not contain data but can declare property. There can be no implementation. Interface can be derived from another interface

Difference between Abstract class and interface

In an interface class, all methods are abstract - there is no implementation.  In an abstract class some methods can be concrete.  In an interface class, no accessibility modifiers are allowed.  An abstract class may have accessibility modifiers