abstract VS. virtual explained

Something I struggled with for a while when first getting into C# was the difference and use of the 'abstract' and 'virtual' keywords. Here is my simple explanation:

abstract - Members modified by this keyword MUST be defined in an abstract class and be implemented on any concrete (non-abstract) class that extends the class the member is defined on. (This corresponds to the 'MustOverride' keyword in VB.NET.)

virtual - Members modified by this keyword CAN be overriden on a concrete class that extends the class that defines the member. (This corresponds to the 'Overridable' keyword in VB.NET.)

C# Example:

abstract class AbstractClass
{
 protected void NotModifiedMethod() { }
 protected abstract void AbstractMethod();
 protected virtual void VirtualMethod() { }
}

class ConcreteClass : AbstractClass
{
 //This HAS to be overriden because it's abstract
 protected override void AbstractMethod() { }

 //This CAN be overriden because it's virtual
 protected override void VirtualMethod()
 {
  base.VirtualMethod();
 }

 //You CAN NOT do this, because NotModifiedMethod is not
 //marked as abstract or virtual
 protected override void NotModifiedMethod() { }
}

VB.NET Example:

MustInherit Class AbstractClass

 Protected Sub NonModifiedMethod()
 End Sub

 Protected MustOverride Sub MustOverrideMethod()

 Protected Overridable Sub OverridableMethod()
 End Sub

End Class

Class ConcreteClass
 Inherits AbstractClass

 'This HAS to be overriden because it's marked MustOverride
 Protected Overrides Sub MustOverrideMethod()
 End Sub

 'This CAN be overriden because it's marked Overridable
 Protected Overrides Sub OverridableMethod()
  MyBase.OverridableMethod()
 End Sub

 'You CAN NOT do this, because NotModifiedMethod is not
 'marked as MustOverride or Overridable
 Protected Overrides Sub NonModifiedMethod()
  MyBase.NonModifiedMethod()
 End Sub

End Class