How do Web sites use cookies?

Cookies evolved because they solve a big problem for the people who implement Web sites. In the broadest sense, a cookie allows a site to store state information on your machine. This information lets a Web site remember what state your browser is in. An ID is one simple piece of state information -- if an ID exists on your machine, the site knows that you have visited before. The state is, "Your browser has visited the site at least one time," and the site knows your ID from that visit.

Web sites use cookies in many different ways. Here are some of the most common examples:

Sites can accurately determine how many people actually visit the site. It turns out that because of proxy servers, caching, concentrators and so on, the only way for a site to accurately count visitors is to set a cookie with a unique ID for each visitor. Using cookies, sites can determine how many visitors arrive, how many are new versus repeat visitors and how often a visitor has visited. Sites can store user preferences so that the site can look different for each visitor (often referred to as customization). For example, if you visit msn.com, it offers you the ability to "change content/layout/color." It also allows you to enter your zip code and get customized weather information. When you enter your zip code, the following name-value pair gets added to MSN's cookie file:

WEAT  CC=NC%5FRaleigh%2DDurham®ION=  www.msn.com/
  • Since I live in Raleigh, N.C., this makes sense.
  • Most sites seem to store preferences like this in the site's database and store nothing but an ID as a cookie, but storing the actual values in name-value pairs is another way to do it (we'll discuss later why this approach has lost favor).

E-commerce sites can implement things like shopping carts and "quick checkout" options. The cookie contains an ID and lets the site keep track of you as you add different things to your cart. Each item you add to your shopping cart is stored in the site's database along with your ID value. When you check out, the site knows what is in your cart by retrieving all of your selections from the database. It would be impossible to implement a convenient shopping mechanism without cookies or something like them.

­ In all of these examples, note that what the database is able to store is things you have selected from the site, pages you have viewed from the site, information you have given to the site in online forms, etc. All of the information is stored in the site's database, and in most cases, a cookie containing your unique ID is all that is stored on your computer.