Web bug
Data-gathering code embedded in a web page or email
A web bug is a piece of code, generally a small file such as a tiny, transparent GIF image, which is used to track data on those viewing the page or mail in which it is hidden.
The bug is in fact a link to the file hosted on a separate server. Thus, if a bugged page is visited, or a bugged email opened, the file will be fetched from the server, providing it with data on which machines are viewing the page or mail, when, and so on. The technique is often used by spammers to verify email addresses, as accessing a particular bug can be proof that an address is live and mails being viewed.
While the use of bugs on websites can have legitimate purposes, the transfer of data to servers other than those visited is often seen as an unnecessary invasion of privacy, and many browsers offer functionality to block such activity. Mail clients can also be configurated to disable bugging, and simply viewing all emails as plain text will prevent bugs from functioning as intended.
