Server Components aren't just 'components that render on the server.' They represent a fundamental shift in how React thinks about the boundary between data fetching and UI rendering. The key insight: most of your components don't need interactivity. They fetch data and return markup. Why ship JavaScript for that?

The Mental Model

Think of your component tree as two interleaved trees: a server tree and a client tree. Server components can import client components, but not vice versa. Data flows down from server to client as serialized props. The 'use client' directive marks the boundary — everything below it ships to the browser.