Abstraction

Relax. We'll talk through this in plain words — no textbook voice.

Abstraction means hiding the implementation details nobody needs and only exposing the features the user actually cares about. In C#, you can use abstract classes and abstract methods to set up a common structure.

csharp

abstract class PaymentMethod
{
    public abstract void Pay(decimal amount);

    public void PrintReceipt(decimal amount)
    {
        Console.WriteLine($"Receipt amount: {amount}");
    }
}

class CardPayment : PaymentMethod
{
    public override void Pay(decimal amount)
    {
        Console.WriteLine($"Paid by card: {amount}");
    }
}

class Program
{
    static void Main(string[] args)
    {
        PaymentMethod payment = new CardPayment();
        payment.Pay(25.50m);
        payment.PrintReceipt(25.50m);
    }
}

How it works

PaymentMethod is an abstract class, so you can't create an object from it directly.
Pay method must be overridden by every derived class — there's no way around it.
PrintReceipt is a common method that every derived class can use.

You should see

Paid by card: 25.50 Receipt amount: 25.50