Abstract class vs. Interface

An abstract class can have abstract members as well non abstract members. But in an interface all the members are implicitly abstract and all the members of the interface must override to its derived class.
An example of interface:

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interface iSampleInterface

{

  //All methods are automaticall abstract

  int AddNumbers(int Num1, int Num2);

  int MultiplyNumbers(int Num1, int Num2);

}

Defining an abstract class with abstract members has the same effect to defining an interface.
The members of the interface are public with no implementation. Abstract classes can have protected parts, static methods, etc.
A class can inherit one or more interfaces, but only one abstract class.

Abstract classes can add more functionality without destroying the child classes that were using the old version. In an interface, creation of additional functions will have an effect on its child classes, due to the necessary implementation of interface methods to classes.