What is a Groundspeak Travel Bug? 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.
(Taken from the Groundspeak webpage describing Travel Bugs)

I love Travel Bugs (TBs)! I love the idea of TBs, the creativity used by people in making them, the generosity of spirit of those who move them along on their missions and take pictures of them en route, and I love tracking those TBs that have crossed my path and continue on their way. TBs are a fabulous addition, or sidebar, to geocaching and my geocaching experience.

That being said, my favorite TB tag will not be moved from cache to cache by anyone other than me, will never fulfill a creative and clever mission like other TBs I've seen (and released), will never have its picture taken (by anyone else), and will not be tracked by people who come across it in a cache and helped to it move along...that's because its my geocaching-backpack. My personal TB accompanies me on every geocaching trip I make (yes, even to events, maintenance trips to my own caches, and caches I never find). I log it into and out of each cache or event I go to, every time I visit.

I shoved the TB tag into a zippered pocket of my geocaching-backpack over 7 months and 5000 miles of geocaching ago. My personal TB has accompanied me to all sorts of caches: from simple to difficult, micro to huge, sub-zero snow-swept woods to blistering heat in blinding desert, ocean to mountain to swamp to red-rock, bone-dry to underwater. My geocaching-backpack has been there when I hunted fruitlessly for DNFs as well as when I have visited my own caches repeatedly to maintain them. It acts as observer, historian, and logbook at each and every stop along my personal geocaching journey.

I sometimes enter only a word or two, sometimes a couple of sentences, but I always log something to remind me of the day and the cache and what was special about the visit. A personal TB is a journal of a person's geocaching history, and is a great way to help you remember the great places you've visited, people you've met, sights you've seen, sunrises and sunsets, open valleys and dense forests, and ultimately...the caches you've found or failed to find. It's also a great way to record the black-fly farms you visited, mud that was deeper than your boots, a caching trip when you (and the TB-backpack) flipped a canoe and tumbled into a pond during a light snowfall (on Memorial Day!), that hike when you took the wrong trail and had to use duct-tape on blisters when you realized you hadn't restocked your 1st aid kit with moleskin.

Does it keep a totally accurate log of my geocaching mileage? No...but it does a great job of helping me look back on some great days of geocaching; it's some of the best money I've spent since I started geocaching.