I saw a question today which was closed because it was "not a question". I do think, that it's incomplete and vague but I think it's not a bad question at all. The question indicates that the poster is just started using .NET and possible OO development and doesn't fully understand the difference between abstract classes and interfaces. I was in the same shoes back then, when I learned .NET from book (and grasped OO for the first time). I think there is a legitimate answer for this question: that the difference between them is marginal (if there is - but could be as they don't compile to the same IL code) and comparing their performance is meaningless. They could be used for a similar reason (e.g. heterogenous collections), but this is not the essence of using them: it's not random that they are different language elements. I think this has more educational value, than closing a question with comments that are not pointing to the right direction - the point of SO is knowledge transfer.
Edit: The question was deleted after I posted my message, I don't know by who and why.