What is React Server Components?

React Server Components (RSC) are a new paradigm (stable in React 19 / Next.js App Router) where components render exclusively on the server and send only the result (serialized React tree) to the client — not JavaScript. Benefits: (1) Zero bundle cost — server component code is never sent to the client. (2) Direct server access — query databases, read files, call backend services directly without API routes. (3) Streaming — server components stream output progressively, enabling instant loading states. (4) Automatic code splitting — only client-interactive parts are sent as JavaScript. Rules for server components: cannot use browser APIs, cannot use hooks, cannot use event listeners, cannot maintain interactive state. Client components (marked with "use client") work like traditional React components — they have state, hooks, and event handlers. Composition: server components can render client components, but client components cannot directly render server components (they can receive them as children). RSC is a fundamental shift in how React apps handle data fetching and rendering.