A payment gateway is a system that allows consumers to send payments to businesses securely and easily. A payment gateway can be a web service, a software solution, or even a physical device. It come in all shapes and sizes. Some are open source and available for anyone to use, while others are proprietary and require access to certain APIs and data sets. Payment gateways also vary in functionality based on the type of transaction they're facilitating: some allow users to make payments using credit cards or debit cards, while others only accept payments through cryptocurrency (like Bitcoin).