A computer cookie is a small text file which contains a unique ID tag, placed on your computer by a website. The website saves a complimentary file with a matching ID tag. In this file various information can be stored, from pages visited on the site, to information voluntarily given to the site. When you revisit the site days or weeks later, the site can recognize you by matching the cookie on your computer with the counterpart in its database.

There are two types of cookies: temporary and permanent. Temporary cookies, also called session cookies, are stored temporarily in your browser's memory and are deleted as soon as you end the session by closing the browser. Permanent cookies, also called persistent cookies, are stored permanently on your computer's hard drive and, if deleted, will be recreated the next time you visit the sites that placed them there.

Cookie technology addressed the need to keep track of information entered at a site so that if you submitted a registration form for example, the site could associate that information with you as you traveled through the site's pages. Otherwise, every time you clicked on a different page in the site, establishing a new connection, the site would lose the information in reference to you, and would have to ask you for it again.

A temporary cookie solved this problem in the short term by setting aside a little bit of your browser's memory to make a "folder" to save information for you. But temporary cookies were lost as soon as you closed your browser. You were not recognized on subsequent visits.

Persistent cookies solved this problem. They allowed a site to recognize you permanently by transferring a text file to your computer with a unique ID tag, matching a complimentary file on the server. Now cookies could persist for years.

Both temporary and permanent cookies can be used for many helpful purposes. Automatic registration log-on, preserving website preferences, and saving items to a shopping cart are all examples of cookies put to good use.

But permanent cookies resulted in unanticipated uses as well.

Many websites began keeping track of when an individual visited, what pages were viewed, and how long the visitor stayed. This information was stored in the visitor's cookie. When he returned, the log of previous visits to the site was immediately known, and the new visit was added to his log. If the visitor ever offered personal information at the site, his real identity, address and other personal information was associated with the anonymous ID tag. Website profiling was born.

Marketers had an even more unique advantage. A given marketer may have advertising rights on several hundred or even many thousands of the most popular websites. In this way the marketer can pass cookies to surfers on countless sites, then recognize a surfer's unique ID tag whenever he or she visits one of their affiliated sites. In this way the marketer can track someone across the web, from site to site, logging a comprehensive profile of the individual's surfing habits over a period of months and even years. Sophisticated profiling programs then sort the data provided by the cookie to categorize the target in several different areas, based on statistical data. Gender, race, income level, political leanings, religious affiliation and even sexual orientation can all be determined with various degrees of accuracy through cookie profiling. Much depends on how much a person surfs, and where they choose to go online.

As a result of public outcry in response to surreptitious profiling, cookie controls were placed in post 3.x browsers to allow users to turn cookies off -- options that were not available in 1995 when permanent cookie technology was first embedded into browsers without public awareness or knowledge of how they could be used. Third-party cookies often have their own controls, as they are normally cookies placed by marketers that are used for profiling.

Cookie controls also allow user-created lists for exceptions, so that one can turn cookies off, for example, but exempt sites where cookies are put to a useful purpose.

The name "cookie" comes from fortune cookie, because of the hidden information inside.