Question : Abstract class and interface

I came across tons of articles defining difference between interface and Abstract class. But I have one particular question: Why do we use interface over abstract class? If I can rephrase my question more precisely: why would I use interface in a particular situation and NOT use abstract class?

I also want to ask the same question other way around.

Answer : Abstract class and interface

Hi,
  you can get isome ideas by this one of Great article on this topic:
   http://www.codeproject.com/KB/architecture/Abstract_Interface.aspx