Cookies and sessions are the twin pillars of stateful HTTP communication, enabling web applications to remember users across requests despite the protocol's inherently stateless nature. Cookies store small pieces of data in the browser that are automatically sent with every request, while sessions maintain server-side state tied to a unique identifier. Together, they power authentication flows, shopping carts, personalization, and user tracking—but also introduce significant security risks if misconfigured. Properly implementing cookie attributes (HttpOnly, Secure, SameSite), regenerating session IDs after privilege changes, and defending against attacks like session fixation and CSRF are non-negotiable for production systems. A well-configured cookie is the difference between a secure session and a hijacked one.