Definition
REST (Representational State Transfer) is an architectural style for web services, introduced in 2000 by Roy Fielding. In practice, "REST API" today almost always means an HTTP API that exposes resources at stable URLs and uses the HTTP verbs (GET, POST, PUT, PATCH, DELETE) to act on them, with JSON bodies for data exchange. It is the dominant style for modern web APIs.
Key REST principles: each URL represents a resource (/users/123, not /getUser?id=123), HTTP verbs express intent, and the server is stateless between requests (auth happens per request, no session affinity). REST is less rigid than it sounds in the original paper; most "REST APIs" in the wild take liberties and are closer to "HTTP+JSON APIs" than pure REST.
How SheetLinkWP relates to REST API
SheetLink Forms exposes several REST endpoints on your WordPress site via the WordPress REST API: Two-Way Sync's /wp-json/sheetlinkwp/v1/sheet-update endpoint receives change events from your Sheet, and the integration-test endpoint lets you verify a webhook round-trip from the admin. The SheetLinkWP backend APIs (license activation, entitlement refresh, AI scoring callbacks, GDPR export) are also REST APIs, all following /api/sheetlinkwp/{resource} conventions.