The difference is simple:
An interface is not a class, and cannot be instantiated. It cannot contain any concrete code, only abstract definitions. You can inherit an Interface as well as a single base class, provided you complete the "contract" and implement the methods and properties it requires.
An abstract class is different: you still can't instantiate it - which means you can't say
MyAbstractBase mab = new MyAbstractBase();
but if you derive from an abstract class you cannot derive from any other class at the same time. An abstract class can contain concrete code, which will be used if it isn't overridden in the derived class.
This is a fairly complicated subject, and you should really start with a C# tutorial for the details:
http://csharp.net-tutorials.com/classes/interfaces/[
^] is a good basis.
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