A payment processor, such as Stripe or a provider built into a commerce platform, handles the movement of money between a customer's bank or card and the merchant. Businesses integrate checkout, subscriptions, invoices, and refunds through the processor's API and webhooks.
Good payment integrations keep card data out of the business's own systems, handle failed and disputed payments gracefully, process events idempotently, and reconcile payouts with internal records so the accounting matches what actually settled.