It is possible to insert a very small image that is hosted on another site into the post. That external site can then record the number of times the image is accessed, and possibly more information from the pulling client.
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Originally Posted by wikipedia
These requests typically include the IP address of the requesting computer, the time the content was requested, the type of Web browser that made the request, and the existence of cookies previously set by that server. The server can store all of this information, and associate it with a unique tracking token attached to the content request. |
It's kind of like the old physical 'tracking bugs' spies used during the cold war -- thus, 'web bugs' or 'email bugs'.