In-app purchase (IAP) is a payment someone makes inside a mobile app for something digital: a sticker pack, an extra feature, a level, or ongoing access. The app store runs the charge — on Android, that is Google Play — so your own card form stays off that sale.
A subscription is one shape of IAP: they pay on a schedule until they cancel, while a one-time unlock ends after a single charge. Checkout is the website version, for a cart that may include shipping. A payment gateway and a payment processor move that web payment.
E-commerce sells on your site; IAP sells something that lives in the app. SaaS often bills on the web, though unlocking the same seat inside the app may still have to go through the store. A loyalty program rewards a purchase they already made and does not charge them.
A real-life example of an in-app purchase
An Atlanta parents’ app gives away the weekly activity list. A $2.99 pack of printable cards is a one-time purchase inside the app, and a monthly weekend planner is a subscription sold the same way. The studio website, where a parent pays for a Saturday class, is a different till.
Decide what is free, what is once, and what renews before you draw the pay button. See app monetization and how to plan a business app.