When you go to a Web site, that Web site's server sends a (hopefully unique) number to your Web browser, sometimes known as a cookie or USI (unique session identifier). Every time that Web site is accessed again, for example by clicking a link to another page on the Web site, the browser resends the cookie so that the server knows what to send and where. The cookie makes it faster and easier for customers to move around your Web site.
A shopping cart is created on the server that is tied to the USI and stores the list of items you have placed in your cart. The items stay in the virtual cart until you enter purchase details or the cookie is timed out by the server or your browser.
Cookies are not in themselves dangerous since they cannot read or write to your computer — they only track what you have done on various Web sites and hold that information on the hosting server. The data is analyzed and mined to produce customer profiles to create targeted marketing campaigns. This information is always stored on a server anyway, but information gained using cookies can be sold to advertisers as anonymous profiles to help them define markets on the Internet.
Turning off cookies solves this problem, but can make some Web sites you visit become unusable, especially if you are shopping online. Most cookies time out, meaning that they will close down themselves after a certain period of inactivity on a site. However, some cookies will stay in system memory until actively flushed out by clearing your cache, and these are the ones that are controversial.
Most Web pages today use third-party cookies placed by advertisers who track user activity around their advertisements and over different Web sites. Even when an ad is not clicked, the third-party cookie can be placed on the browser and track its activity. This tracking across multiple Web sites that are not stored on the same server is considered by some to be an invasion of privacy.
The Bottom Line
Sessions are great since they allow your customers to move around your Web site without having to log in or reload pages multiple times. They are also used to collect information about how customers use your Web site, which can be analyzed and parlayed into new and better versions of your Web site.
It is important to scan any affiliate or advertising contracts for permissions to use persistent third-party cookies. You may be eroding consumer confidence as well as giving away extremely valuable marketing profiles on your customers. Shouldn't you be the one profiting from inside information on your customer base?