Web beacon

Web beacon, tracking bug, tag, page tag or web bug is an object that is embedded in a web page or email and is usually invisible to the user but allows checking that a user has viewed the page or email. 

Common uses are email tracking and page tagging for Web analytics.

Common names for web bugs implemented through an embedded image include tracking pixel, pixel tag, 1×1 gif, and clear gif. When implemented using JavaScript, they may be called JavaScript tags.

Web bugging is the use of these techniques, analogous to conventional bugging, though not as invasive or intrusive. The distinction of a web bug should not be confused with the more benign web spider nor the more malicious, nefarious or subversive devices such as computer worms. A web bug should also not be confused with a software bug, which is a glitch or error in a program.

http://www.webopedia.com/TERM/W/Web_beacon.html

http://www.allaboutcookies.org/web-beacons/

http://www.allaboutcookies.org/faqs/beacons.html

http://info.yahoo.com/privacy/us/yahoo/webbeacons/