Web bug
Alternatively called a web beacon, tracking bug, invisible GIF, 1 by 1 GIF, or pixel tag, a web bug is a 1x1 pixel image used to track a user reading a web page or e-mail. After the GIF (graphics interchange format) image loads, the time loaded, how long it was viewed, the IP address, and the browser used can be tracked. For example, the file __utm.gif is a small image file loaded on web pages with Google Analytics to help keep track of page views.
To prevent a web bug from tracking Internet activities, turn off cookies and see if your browser has web bug-blocking add-ons. To prevent a web bug from tracking e-mails, most e-mail clients today warn users when an e-mail is attempts to load images, which lets you accept or prevent images from loading. You can also turn off HTML (hypertext markup language) e-mails or only accept plain-text e-mails.