Paytrail is an payment service provider that you (the consumer) can use to pay for your online purchases. You can store your card details in your account, thus enabling online payments with your login. This saves you time, since you don't need to repeatedly enter your card details when making a payment.
You get login credentials when you create an account within the service.
Note: At the moment, Paytrail account is available only for Finnish customers (in Finnish).