Cookies are small pieces of data stored by web browsers on a user's device at the request of web servers, allowing servers to "remember" specific user information and facilitating functionalities like maintaining user sessions and tracking user behavior. The Set-Cookie response header determines how the browser will treat cookies, with attributes such as Name and Value, Domain and Path, Expires and Max-Age, Secure, HttpOnly, and SameSite defining their scope, expiration, and security settings. Prioritizing security when working with cookies is essential, involving practices like using Secure and HttpOnly attributes, setting appropriate SameSite values, limiting cookie lifespan, avoiding sensitive data, inspecting and debugging cookies to identify potential issues, and utilizing browser developer tools for streamlined cookie management.