What is the difference between interface and abstract class

在国外的一个论坛上看到一个关于抽象类和接口比较的帖子，其中有一篇感觉很有道理：
Interface versus Class Inheritance is pretty easy once you understand it.
There is a very easy way to distinguish the 2.
Class inheritance = implemenation inheritance.
What this means is that if you have a base object you inherit from, your new derived object will either contain or override the existing functionality of the base object. This is often referred to as "is a " relationship.
A Beagle IS A Dog.
Interface inheritance is a little different. It is basically like a contract and is usually referred to as a "has a " relationship. By agreeing to implement an interface, you agree to implement all of its members, and provide your own implementation. An Interface should describe what a class does, rather than what a class is.
A Beagle Has A Bark
One reason to use Interfaces over Implementations is versioning or upgradability.
Any changes you make to your base class (abstract class) could possibly break subclasses that are derived from that class. By implementing interfaces you alleviate this problem by allowing clients or code to have multiple interfaces. By using interfaces, subclass can only see its parents interface, not the implementation.