What are React Server Components (RSCs), and how do they reduce client bundle size?

// Server Component - no JS shipped to client
async function UserList() {
  const users = await db.users.findMany();
  return (
    <ul>
      {users.map(user => (
        <li key={user.id}>{user.name}</li>
      ))}
    </ul>
  );
}

// Client Component - JS only where needed
'use client';
function LikeButton({ postId }) {
  const [liked, setLiked] = useState(false);
  return <button onClick={() => setLiked(!liked)}>Like</button>;