Guide about Cookies

Cookies are a very important method for maintaining state on the Web. "State" in this case refers to an application's ability to work interactively with a user, remembering all data since the application started, and differentiating between users and their individual data sets.

An analogy I like to use is a laundry cleaner's shop. You drop something off, and get a ticket. When you return with the ticket, you get your clothes back. If you don't have the ticket, then the laundry man doesn't know which clothes are yours. In fact, he won't be able to tell whether you are there to pick up clothes, or a brand new customer. As such, the ticket is critical to maintaining state between you and the laundry man.

Unfortunately, HTTP is a "stateless" protocol. This means that each visit to a site (or even clicks within a site) is seen by the server as the first visit by the user. In essence, the server "forgets" everything after each request, unless it can somehow mark a visitor (that is, hand him a "laundry ticket") to help it remember. Cookies can accomplish this.