Cookies are maintained by the browser and are sent in the HTTP request header using the Cookie header field. Each cookie has a name, value, domain, path, expiration time, and some other parameters. When a request is sent to a URL that matches the domain, the path of a non-expired cookie, the client sends the cookie to the server. Cookies are usually stored in small files on the client by the browser or in a local database. The actual implementation is the business of the browser, and we need not worry about it. It is just the text information that is not executed by the client. It is only sent back to the server when some rules (mainly domain and path) match. Cookies are created by servers and are sent to the client in HTTP responses using the Set-Cookie header field. Thus, essentially the server tells the client, Hey, here is this cookie, whenever you come to me next time, show me this piece of information, so I will know it is you.

Cookies are usually to remember clients. Advertisers...