When someone makes a purchase, there is a particular process that happens between them entering their credit card number and money arriving in your bank account. The service that makes that process happen is called a payment gateway, and they compete on availability, price and some other services. The most common are Stripe and PayPal, but there are tons of others available. Shopify has its own payment gateway that waives the fees for Shopify merchants, and it pairs with over 70 payment gateways including virtual currencies and payment instalment providers. See a comparison of top gateways in 2017 right here.