An interface provides a list of members, without an implementation, that a class can choose to implement. This is similar to an abstract class, which may include abstract methods that have no implementation in the abstract class, but might also include an implementation for some of its members.
One difference is that an abstract class might include some members that are fully implemented in the abstract class. Interface can't include the implementations of any of their members. An interface just describes what a class does, while an abstract class may define how something is done.
Another difference is that a class can inherit from multiple interfaces, but can inherit from at most one base class. Abstract classes allow you to treat an object polymorphically, based on any of the classes in its inheritance chain. Interfaces let you treat a class polymorphically, based on any of the interfaces it implements.
原文地址：#445 - Differences Between an Interface and an Abstract Class