What is a cookie?

A cookie is a parcel of textual information sent by a server to a World Wide Web browser and then sent back by the browser each time it accesses that server.

HTTP cookies are used for user authentication, user tracking, and maintaining user-specific information such as site preferences and electronic shopping carts.

Cookies have been of concern for Internet privacy, since they can be used for tracking the browsing of a user. As a result, they have been subject to legislation in various countries such as the United States, as well as the European Union. Cookies have also been criticised because the identification of users they provide is not always accurate and because they can be used for network attacks.

On the other hand, cookies have been subject to a number of misconceptions, mostly based on the erroneous notion that they are computer programs. In fact, cookies are simple pieces of data unable to perform any operation by themselves. In particular, they are neither spyware nor viruses.

Most modern browsers allow users to decide whether to accept cookies, but rejection makes almost any Web site unusable (since almost any dynamic web site relies on per-session cookies to implement session persistence).