What are HTTP cookies?

HTTP cookies, sometimes known as web cookies or just cookies, are parcels of text sent by a server to a web browser and then sent back unchanged by the browser each time it accesses that server.
HTTP cookies are used by Web servers to differentiate users and to maintain data related to the user during navigation, possibly across multiple visits.
Allowing users to log in to a website is another use of cookies. Users typically log in by inserting their credentials into a login page; cookies allow the server to know that the user is already authenticated, and therefore is allowed to access services or perform operations that are restricted to logged-in users.