This document provides an overview of online payment systems and the key entities involved. It describes the ecosystem on both the client and business sides, including credit card holders, issuing banks, acquiring banks, payment processors, merchant accounts, and gateways. It then outlines the basic payment flow from a customer sending their card details through approval, provision of goods/services, and settlement. There are many additional topics to learn about such as chargebacks, security, payment types, and legal requirements.