There are a few special items you will run across in the geocaching world that are "trackable", meaning that the current and past locations of the object are tracked by logging them at the www.geocaching.com website.
If you find one of these items in a cache there are a few responsibilities that come along with removing it.
The trading policy for cache items does not apply to trackable items. You should not keep them, only move them from one cache to another.
Travel Bugs
A travel bug can be almost any object. It always has a "dog tag" attached with a serial number inscribed to help track it. Travel bugs are very special because they have a life of their own. They were set free into the wild by their owner and usually represent some characteristic of the owner. They have an objective. They also have a past history of the caches they have visited and a future - in you, the person that will take them on the next leg of their journey. Visit the travel bug's home page to find our more details about it.
GeoCoins
These are more compact versions of travel bugs and come in a variety of shapes and designs. A serial number is always inscribed somewhere on the coin to help track it.