If you want to accept card payments online, money has to be transferred from the cardholder's account to your merchant account. Banks will not allow you to connect directly to their systems and so it is necessary to have a trusted "payment gateway" between your website and the bank's systems.
All of the authorisation, fraud checks and funds transfers are done through the payment gateway. Payment gateways charge for this service - some charge a flat monthly fee, others charge based on the value of transactions.
We are happy to recommend a suitable payment gateway for use in your country - please get in touch and we will let you know which ones to look at. In the UK we favour Sagepay but also work with other payment gateways.