What are cookies and how do they work?

Cookies are key-value pairs the browser stores and automatically resends with every request to the same domain.

How they work:

1. Server sets a cookie via Set-Cookie header in response
2. Browser stores the cookie
3. Browser sends cookie with every subsequent request to that domain
4. Server can read the cookie values

Cookies persist across requests and browser sessions (until they expire), enabling stateful behavior over stateless HTTP.

Go deeper:

• Using HTTP cookies (MDN) — the canonical reference on Set-Cookie, the Cookie header, and lifetime/security attributes.
• HTTP cookie (Wikipedia) — origin of the name, cookie types, and the privacy/security landscape.