REST
A style for designing web APIs around resources and standard HTTP verbs.
REST treats everything as a resource (a user, an order) addressed by a URL, and you act on them with GET, POST, PUT, DELETE. It's stateless, so each request carries everything the server needs. Its simplicity and use of plain HTTP made it the default way services talk over the web.
True REST includes HATEOAS (hypermedia links), though most 'RESTful' APIs are simpler resource-CRUD over HTTP; statelessness aids scaling but pushes state to the client or tokens.
GET /users/1, POST /orders, status codes 200/404/500, JSON payloads.
Like ordering from a restaurant menu by item number: the kitchen doesn't need to remember your last visit, you just state exactly what you want each time.