I am having trouble finding any documentation on:

1.  What is the difference between Abstract and Virtual classes (they seem to be the exact same to me)

2. What are some scenarios for using Abstract or Virtual classes (wrapper classes comes to mind but are these the right thing to use?)

3.  When would you use an Interface instead of an Abstract or Virtual?  I read they are only suppose to be signatures but I saw an example that had a method with code in it too..