HTTP is said to be a stateless protocol. What this means for web programmers is that every time a user loads a page it is the first time for the server. The server can't say whether this user has ever visited that site, if is he in the middle of a buying transaction, if he has already authenticated, etc.

A cookie is a tag that can be placed on the user's computer. Whenever the user loads a page from a site the site's script can send him a cookie. The cookie can contain anything the site needs to identify that user. Then within the next request the user does for a new page there goes back the cookie with all the pertinent information to be read by the script.

The mod_python Cookie module offers three Cookie classes: Cookie, SignedCookie and MarshalCookie