What is the difference between useState and useEffect in React?

**useState** — manages state (data that can change over time)
**useEffect** — handles side effects (API calls, subscriptions, timers)

```jsx
import { useState, useEffect } from 'react';

function Profile({ userId }) {
const [user, setUser] = useState(null); // useState

useEffect(() => { // useEffect
fetch(`/api/users/`)
.then(r => r.json())
.then(setUser);
}, [userId]); // runs when userId changes

if (!user) return <p>Loading...</p>;
return <h1>{user.name}</h1>;
}
```