Pardon me, but google is your friend.
http://www.worldprivacyforum.org/cookieoptout.html...these kinds of cookies are typically sent to your computer by advertising or marketing companies, and they can last for decades. The reason these cookies can create a long history of your Web browsing and job searching activities is because these kinds of cookies allow companies to track your movements across many different Web sites. This can be a real problem over time.
For example, if you are looking for a job on Monster.com (which as of this writing deposits advertising.com cookies, among others) and then you go look at a health Web site such as MD.com, then a company called Advertising.com knows you have been to both places. That's because Advertising.com puts a tracking cookie, or a file, on your computer to report back to them whenever you visit one of the sites in their advertising network, sometimes even saying what pages or specific jobs you looked at. Some sites even have cookies on areas where you fill out forms or post a resume, so those companies know when you have posted a resume and may know what kind of information you are giving out online.
What do the marketing companies actually learn from tracking cookies? It may surprise you. If you have filled out forms online with your real name and contact information, or have clicked on banners then purchased an item, or if you have filled out sweepstakes or contests forms, then it is quite possible that major online advertisers know your name and have associated it with your Internet Protocol, or IP address and other information........