1) For abstract class a method must be declared as abstract. Abstract methods doesn’t have any implementation.
For interface all the methods by default are abstract methods only. So one cannot declare variables or concrete methods in interfaces.

2) The Abstract methods can declare with Access modifiers like public, internal, protected. When implementing in subclass these methods must be defined with the same (or a less restricted) visibility.
All methods declared in an interface must be public.

3)Abstract class can contain variables and concrete methods.Interfaces cannot contain variables and concrete methods except constants.

4)A class can Inherit only one Abstract class and Multiple inheritance is not possible for Abstract class.
A class can implement many interfaces and Multiple interface inheritance is possible.