Simply put, a Groundspeak Travel Bug is a trackable tag that you attach to an item. This allows you to track your item on Geocaching.com. The item becomes a hitchhiker that is carried from cache to cache (or person to person) in the real world and you can follow its progress online.
Each Travel Bug comes as a pair; One bug gets attached to an item (or on its own) and the other is a copy that you can hold on to. The Travel Bug tags has its own unique tracking number stamped on it. This tracking number is used as proof by the user that they found the item. It also doubles as a way for the user to locate the personal web page for the travel bug.