A payment gateway is a service that securely passes payment details from a merchant’s page to the acquirer and returns the result of the payment.
In plain words
An online shop does not handle card data itself: the buyer is redirected to the gateway’s page (or the gateway’s form is embedded), where the details are entered. The gateway connects the site, the acquirer and the payment system and returns a status to the shop: “successful” or “declined”. Some gateways connect several banks and payment methods; they then charge their own fee on top of the acquiring fee. That is why you should compare the total cost in a tariff.
Example. The buyer clicks “Pay” and a gateway form with the acquiring bank’s logo opens. After entering the details and passing 3-D Secure, the site receives the answer and shows “Thank you for your order”.