An abstract method is a method that must be implemented to make a concrete class. The declaration is in the abstract class (and any class with an abstract method must be an abstract class) and it must be implemented in a concrete class.
A virtual method is a method that can be overridden in a derived class using the override, replacing the behavior in the superclass. If you don't override, you get the original behavior. If you do, you always get the new behavior. This opposed to not virtual methods, that can not be overridden but can hide the original method. This is done using the new modifier.
See the following example:
public class BaseClass
{
public void SayHello()
{
Console.WriteLine("Hello");
}
public virtual void SayGoodbye()
{
Console.WriteLine("Goodbye");
}
public void HelloGoodbye()
{
this.SayHello();
this.SayGoodbye();
}
}
public class DerivedClass : BaseClass
{
public new void SayHello()
{
Console.WriteLine("Hi There");
}
public override void SayGoodbye()
{
Console.WriteLine("See you later");
}
}
When I instantiate DerivedClass and call SayHello, or SayGoodbye, I get "Hi There" and "See you later". If I call HelloGoodbye, I get "Hello" and "See you later". This is because SayGoodbye is virtual, and can be replaced by derived classes. SayHello is only hidden, so when I call that from my base class I get my original method.
Abstract methods are implicitly virtual. They define behavior that must be present, more like an interface does.