I've seen mention of travel bugs (or trackables) on geocaching sites. What is a travel bug and what can I do with one?
A travel bug (or trackable) has a alphanumeric code that can be entered on the geocaching.com website to track its progress as it moves from cache to cache. You can purchase and enter your own codes or just help move someone else's along.
When you find one you enter the code at the website and leave a short note about where you found it or future plans. At the site you can see a listing of the owner's goal for the travel bug (move around the world, reach a specific destination, etc). Then next time you find a suitable geocache you can drop the trackable for the next person to find.
The trackable is the object which can be moved from cache to cache and tracked with help of tracking code. This is something like normal exchange object, but you can track it and know who have taken it and where have it landed. There are only other rules for trackables as for exchange objects - you can't trade trackable for normal exchange object, but you can take as many trackables as you wish, only that you must put then to the other cache and not keep it.
The travel bugs is only the one system of trackables, operated by geocaching.com. This is a commercial system, you must pay for trackable identifiers.
There are other trackable systems, the most popular from them is now geokrety. This system is free, you pay absolutely nothing for registering as many codes as you wish, only you must print the tracking labels yourself.