Web bugs' (a specific method of delivering a crafted web page element) are a sub-class of 'spyware', which in turn is a subclass of malware.
Web beacons, bugs, and 1X1 pixels
"A Web Bug is a graphics on a Web page or in an Email message that is designed to monitor who is reading the Web page or Email message. Web Bugs are often invisible because they are typically only 1-by-1 pixel in size. They are represented as HTML IMG tags.
Ad networks can use Web Bugs to add information to a personal profile of what sites a person is visiting. The personal profile is identified by the browser cookie of an ad network. At some later time, this personal profile which is stored in a data base server belonging to the ad network, determines what banner ad one is shown.
Another use of Web Bugs is to provide an independent accounting of how many people have visited a particular Web site.
Web Bugs are also used to gather statistics about Web browser usage at different places on the Internet.
Bugs can get:
* The IP address of the computer that fetched the Web Bug
* The URL of the page that the Web Bug is located on
* The URL of the Web Bug image
* The time the Web Bug was viewed
* The type of browser that fetched the Web Bug image
* A previously set cookie value