A pathtag is a small (1" diameter) coin that is left as signature items by some geocachers. The design on the face of the coin is customized, while the other side holds a generic design and a serial number. They're often mistaken for geocoins, but there are three main differences between geocoins and pathtags. The most important difference is that pathtags have nothing to do with GroundSpeak and are not trackable at geocaching.com. Another difference is that the tags are not individually trackable - all tags of the same design have the same serial number. While it is possible to track where the tags were logged, it is not possible to track the path of an individual tag. Finally, tags are usually meant to be kept by the finder, instead of being moved like activated geocoins.
A pathtag has an iron core, plated, with a custom design on one side. This gives it another property that is different from most geocoins - it is magnetic, and if the plating is compromised, it will rust. The design can be done by the purchaser of the tags, or professionally designed for a small fee by 3rd party sites.
For more information, see http://pathtags.com