What is the difference between an abstract function and a virtual function?

An abstract function cannot have functionality. You're basically saying, any child class MUST give their own version of this method, however it's too general to even try to implement in the parent class.

A virtual function, is basically saying look, here's the functionality that may or may not be good enough for the child class. So if it is good enough, use this method, if not, then override me, and provide your own functionality.

An abstract function has no implemention and it can only be declared on an abstract class. This forces the derived class to provide an implementation.

A virtual function provides a default implementation and it can exist on either an abstract class or a non-abstract class.

So for example:

public abstract class myBase
{
    //If you derive from this class you must implement this method. notice we have no method body here either
    public abstract void YouMustImplement();

    //If you derive from this class you can change the behavior but are not required to
    public virtual void YouCanOverride()
    { 
    }
}

public class MyBase
{
   //This will not compile because you cannot have an abstract method in a non-abstract class
    public abstract void YouMustImplement();
}

1. Only abstract classes can have abstract members.
2. A non-abstract class that inherits from an abstract class must override its abstract members.
3. An abstract member is implicitly virtual.
4. An abstract member cannot provide any implementation (abstract is called pure virtual in some languages).