Polymorphism

Method overriding

public class Animal
{
  public virtual void Walk()
  {
    // default implementation
  }
}

public class Person : Animal
{
  public override void Walk() {}
}

virtual in the base class indicates the method can be overrided.

override means the method is overrided.

Abstract class

public abstract class Animal
{
  public abstract void Walk();
}

public class Person : Animal
{
  public override void Walk() {}
}

Sealed

sealed prevents derivation of a class and overriding of a method.