Cookies

Small, server-set text files stored on the client (browser); they manage state for the stateless HTTP protocol, enabling session tracking and user personalization.

Cookies are critical for maintaining state across the inherently stateless HTTP protocol. A web server uses the `Set-Cookie` response header to send a small key-value data pair (e.g., `sessionID=a3fWa`) to the user's browser. The browser then automatically attaches this data in a `Cookie` request header on all subsequent requests to that domain. This mechanism is essential for core web functions: managing authenticated user sessions, remembering shopping cart contents (e-commerce), and storing user preferences (like dark mode settings). They are categorized as session, persistent, first-party, or third-party, each with distinct security implications (e.g., `HttpOnly` and `Secure` attributes).

https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies