Think about this every time you forward a message... jokes, virus warning, biblical quotes, prayers to pass on, Obama presentations, cute sayings, nice pictures, etc.
The originator places a 'web bug' in the message. Also called a "Web beacon," "pixel tag," "clear GIF" and "invisible GIF," it is a method for passing information from the user's computer to a third party Web site. Used in conjunction with cookies, Web bugs enable information to be gathered and tracked in the stateless environment of the Internet. The Web bug is typically a one-pixel, transparent GIF image, although it can be a visible image as well. As the HTML code for the Web bug points to a site to retrieve the image, it can pass along information at the same time.
Web bugs can be placed into an HTML page used for e-mail messages as most mail programs support the display of HTML pages.
The Web bug is inserted into the message, which is an invisible GIF with a unique name obtained from the tracker's server. When the recipient previews or opens the message, the GIF is downloaded from the tracker's server, which reads the name and notifies the sender.
The information relayed can include your email address, ISP, IP address, etc.
Again, this is called social engineering, which is described in a previous post.
So be aware of these threats, especially when forwarding email of which you have no idea who the 'original' originator is.