Web Bug/Beacon:

A web bug also referred to as a web beacon is a file object, usually a transparent graphic image. A web bug can come unnoticeably through an email or it can be on a web page one may access while online. A web bug or beacon is a type of spy ware. It's used to monitor the things a user does online. It's used a lot by spammers to confirm the email addresses of users. Once the user opens a email sent by a spammer, the web beacon sends information back to the sender that way they know you opened the email.