Web bug

Similar to a cookie, it is an invisible file hidden on a Web page or in an HTML e-mail to help marketers determine who has seen their ads.

Whereas cookies are tiny text files placed on your computer hard drive to help marketers track your movements and to help users personalize Web sites, Web bugs are buried in the source code as tiny IMG (image) tags that can supply the merchants with your IP address and match a browser cookie to your e-mail address (so they'll know who you are if you visit their Web sites).