implementation...

a virtual member (property / method) defines a body / implementation which inheritors can override.

an abstract member does not define a body / implementation and inheritors *must* implement them. also, abstract members can only be defined in abstract classes.

Frankly speaking, I found no "real" difference. When in both cases you ultimately override and change the definition in subclass, what's the use of defining it earlier?
It's bit of juggling the code?

there's a HUGE difference in a number of ways. virtual provides implementation to inheritors which does not have to be implemented by the inherited class where abstract doesn't. when you inherit from an abstract class (much like an interface) you are *required* to provide an implementation. inside an overridden abstract calling base.OverriddenMethod does nothing (and I wonder if you wouldn't get a compile error). the base class does nothing other than define the contract whereby each inheritor fulfill.

when creating a set of classes that can be inherited, extended and modified, you will want to share common implementation. there will also be functionality which will end up being implementation specific.

a good example of this is a sort algorithm. if you have a base sort class you wouldn't want to implement a sort algorithm as they're all implementation specific:

public abstract class SortAlgorithmBase {

    public abstract Array Sort(Array array);

}



public interface ISortAlgorithm {

   Array Sort(Array array);

}