Travel Bugs (a.k.a. TB) are items left in a cache that are not normal trade items. TB have an owner who is also a geocacher, and they have attached a TB "dogtag" to some item of interest (a.k.a. hitchhikers) for the purpose of tracking its movements.
The TB dogtag contains a serial number which is used to track the TB on the internet. These numbers should not ever be publicized in logs so that the bug is not tampered with maliciously. When you find a TB, visit the Travel Bug webpage to log your find.
TB sometime have specific goals, and if you find one it is a good idea to read any attached documentation to see if you can help if to a goal. If you pick up a TB you should help it achieve its goal if possible. To keep the TB moving try and drop them into another cache within two weeks, and always try to pick a cache that is not as susceptible to being found by geomuggles; TB are an investment and owners are disappointed when they are lost.
There are several different types of travel bugs. The most common is the dog tag type. These tags can be purchased from Groundspeak. There are also geocoins that can also be tracked through the same webpage as the dog tags. There are other variety of coins that you may find, but only the USA Geocoins and Moun10Bike coins can be tracked through Groundspeak. The others can probably be tracked through their own web pages, examples of these are the Canadian coins and the Texas coins. There are a variety of coins in circulation, click here to see some of them.
If you decide that you would like to track a travel bug you should understand that placing it in circulation always risks it being stolen. The fun of watching a bug travel is worth the risk for many people, it is much more fun than just holding it in your hand :)
When choosing a hitchhiker to attach your travel but to, keep in mind that size is important. Too large of a hitchhiker will not easily fit into caches which may slow its progress.
Travel Bugs should be placed in a zip lock bag to protect them and to help them to be recognized as something other than a normal trade item. Choose a high quality bag, they last longer and are worth the extra cost. It is also a good idea to include a copy of the listing with the travel bug so that people can read about it's goal. The smaller the travel bug the more important it is to place it in a bag with a paper description. Small travel bugs like geocoins can easily get lost if not distinguished this way.