When to Use Interface and Abstract Class?

1.) http://msdn.microsoft.com/en-us/library/scsyfw1d%28v=vs.71%29.aspx
2.) http://adavesh.blogspot.in/2012/06/interface-vs-abstract-class-why-and.html
3.) https://www.youtube.com/watch?v=yyU3bXyc_oU
4.) http://csharp-video-tutorials.blogspot.in/2013/09/part-3-why-and-when-should-we-use.html