A cookie (also tracking cookie, browser cookie, and HTTP cookie) is a small piece of text stored on a user's computer by a web browser. A cookie consists of one or more name-value pairs containing bits of information.

The cookie is sent as an HTTP header by a web server to a web browser and then sent back unchanged by the browser each time it accesses that server. A cookie can be used for authentication, session tracking (state maintenance), storing site preferences, shopping cart contents, the identifier for a server-based session, or anything else that can be accomplished through storing textual data.

As text, cookies are not executable. Because they are not executed, they do not qualify as spyware or viruses. Anti-spyware products may warn users about some cookies because cookies can be used to track people or violate privacy concerns.

Most modern browsers allow users to decide whether to accept cookies, and the time frame to keep them, but rejecting cookies makes some websites unusable.