В чем разница между unit и интеграционным тестированием

@Test
void testCalculateDiscount() {
    OrderService service = new OrderService();
    double discount = service.calculateDiscount(1000);
    assertEquals(100, discount);
}

@Test
void testOrderProcessing() {
    OrderService service = new OrderService();
    PaymentGateway gateway = new RealPaymentGateway();
    Order order = service.createOrder(1000);
    boolean result = gateway.processPayment(order);
    assertTrue(result);
    assertNotNull(order.getPaymentId());
}