Cookie

From Baruchnewmedia

Cookie is a small file that holds information about you and your web activities, which a web site places on your computer. Cookies help advertisers target ads to individual Facebook users, because these cookies reportedly could automatically identify each Facebook user as they surf the web, and then serve advertising relevant to their stated interests on Facebook.

http://en.wikipedia.org/wiki/HTTP_cookie

http://venturebeat.com/2007/10/29/cookie-tracking-how-facebook-could-be-worth-100-billion/