Definitions for "Travel Bug"
A travel bug is an item that is placed in a cache that has a traceable ID tag. Anyone is welcome to take a travel bug from a cache, and if you do take it with you, you should log online at http://www.geocaching.com/track/ and record that you picked it up. Shortly thereafter, you should try and place the travel bug in a different cache. Again you'll want to log online to record where you placed it. You can generally tell on geocaching.com, which caches currently have travel bugs located in them by the bug icon shown on the cache's page. The progress (movement) of travel bugs can be followed at http://www.geocaching.com/track/.
a form of hitchhiker you can track on Geocaching
a Geocaching tracking tag