There are 4 main players involved in a card transaction:
In order to make a card transaction possible, the merchant needs a payment gateway.
A payment gateway functions as a POS terminal but a virtual one. In brief, it is a solution for transferring transaction data to the payment processor which forwards it to a cardholder’s issuing bank and receiving a response (authorization approved or declined). More information can be found on a separate page dedicated to a payment gateway.
This information can also be accessed as a PDF document. Feel free to download.