What are Shopify Functions and when do I use them over an app?

Shopify Functions let you customize backend logic, custom discounts, checkout validation, payment/shipping customizations, cart transforms. That run on Shopify's infrastructure, fast and close to checkout. They're how you change the rules of checkout (especially on Plus) without bolting on heavy front-end scripts.

They're usually delivered as part of an app, but the point is the logic runs natively rather than as client-side JavaScript, so it's fast and reliable. Reach for Functions when you need custom checkout/discount behavior; reach for a regular app when you need UI, dashboards, or integrations.