What is: Web beacon

Also called a Web bug or a pixel tag or a clear GIF. Used in combination with cookies, a Web beacon is an often-transparent graphic image, usually no larger than 1 pixel x 1 pixel, that is placed on a Web site or in an e-mail that is used to monitor the behavior of the user visiting the Web site or sending the e-mail. When the HTML code for the Web beacon points to a site to retrieve the image, at the same time it can pass along information such as the IP address of the computer that retrieved the image, the time the Web beacon was viewed and for how long, the type of browser that retrieved the image and previously set cookie values.

Web beacons are typically used by a third-party to monitor the activity of a site. A Web beacon can be detected by viewing the source code of a Web page and looking for any IMG tags that load from a different server than the rest of the site. Turning off the browser's cookies will prevent Web beacons from tracking the user's activity. The Web beacon will still account for an anonymous visit, but the user's unique information will not be recorded.

Source: Webopedia.com (part of internet.com) & Allaboutcookies.org