What is the difference between abstraction and encapsulation?

Abstraction is about hiding what something does — exposing a simple interface that omits implementation details.

public interface IPaymentGateway {
    Task<PaymentResult> ChargeAsync(decimal amount, string token);
}

The caller doesn't know if it talks to Stripe, Braintree, or a mock. That's abstraction.

Encapsulation is about hiding how something works — protecting internal state from outside mutation.

public class BankAccount {
    private decimal _balance;                       // hidden state
    public decimal Balance => _balance;             // controlled read
    public void Deposit(decimal amount) {           // controlled write
        if (amount <= 0) throw new ArgumentException();
        _balance += amount;
    }
}

You can't bypass Deposit and assign _balance directly. That's encapsulation.

Quick mnemonic:

• Abstraction → "I don't need to know HOW the payment is processed"
• Encapsulation → "You can't reach in and break my invariants"

Both serve the same goal (reducing accidental complexity) but at different layers — abstraction at the API boundary, encapsulation inside the class.