What is GraphQL and how does it differ from REST in Node.js?

GraphQL is a query language for APIs and a runtime for executing those queries, developed by Facebook. Unlike REST which has fixed endpoints returning fixed data shapes, GraphQL exposes a single endpoint (/graphql) where clients specify exactly what data they need. Key differences: (1) Over-fetching: REST often returns more data than needed; GraphQL returns exactly what was requested; (2) Under-fetching: REST may require multiple requests for related data; GraphQL fetches all in one request; (3) Schema: GraphQL has a strongly typed schema that serves as a contract; (4) Versioning: REST often needs /v1, /v2; GraphQL evolves by adding fields (deprecated ones remain). Node.js implementations: Apollo Server (most popular), graphql-yoga, Mercurius (for Fastify). Best suited for: complex data graphs, multiple clients (mobile/web) with different data needs, rapid product iteration. REST is still better for simple CRUD, file uploads, and when HTTP caching is important.