React Server Components

React Server Components are a class of components that execute exclusively on the server. They can read from databases, hit APIs, and render JSX without any of their JavaScript reaching the browser bundle. The browser receives the rendered output as a serialized stream the React runtime can splice directly into its tree alongside ordinary client components.

The practical wins are smaller bundles (libraries used only on the server stay there) and easier data fetching (a component can simply `await db.query(...)` and render the result, no API endpoint required). The cost is a clear separation between server work and client interactivity — components that need state or effects must opt into client rendering with a `"use client"` directive.

Next.js App Router is built around RSC; every page and layout is a server component by default unless marked otherwise.