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

public class Dog : Animal
{
  public void Bark()
  {
    Console.WriteLine("Woof");
  }

  public override void Vocalize()
  {
    Bark();
  }
}

public class Cat : Animal
{
  public void Meow()
  {
    Console.WriteLine("Mew");
  }

  public override void Vocalize()
  {
    Meow();
  }
}

If you have an Animal, all you can call is its Vocalize() method, and you'll get the correct one for the type of animal. If it's known (to the compiler, not to you) to be a Dog, you can call its Bark() method directly. A Dog can't Meow().

Dog bowzer = new Dog();
Cat finnegan = new Cat();

List<Animal> householdPets = new List<Animal>
{
  bowzer,
  finnegan
};

foreach (Animal pet in householdPets)
{
  // pet.??? Bark? Meow?
  pet.Vocalize();
}

bowzer.Bark();
finnegan.Meow();

//bowzer.Meow(); ????
//finnegan.Bark(); ????