A Travel Bug is a trackable tag that you attach to an item and looks similar to a dog tag. A travel bug can be attached to another item by use of the chain on the travel bug if required, examples of such items include cuddly animals, toy cars, or keyrings.
Early geocachers started to leave personal signature items such as wood tokens, bottle tops etc. in geocaches they found. One unknown US military personal had their own "challenge coin" made, similar to the popular challenge coins used the in military and started leaving copies in geocaches. Others soon followed and these new coins quickly became known as Geocoins. A forward thinking geocacher then had the idea of making geocoins trackable by stamping a code on them so they could travel from cache to cache.
Following this the first geocoins were developed by Jon Stanley (aka moun10bike) as a signature item to be placed in caches that could be tracked 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 or geocoin 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.
Travel Bug Trackables are tracked with the help of users who go online and 'grab' them from caches, or receive them from users. The idea is by picking up and dropping off Travel Bug Trackables on the web site you are mirroring the Bug's real world adventures. Each Travel Bug has its own 'diary' that follows its movements.
The owner of the travel bug or geocoin gives it a mission or task that they desire. Or no task at all. These missions can be as simply as "to travel as far as possible" or to travel to a specific cache, location or a certain type of location. The fun of a Travel Bug is inventing new goals for the Travel Bug to achieve. One Bug's goal may be to reach a specific country, or travel to 10 countries.
A travel bug meeting one of its goals at Glasgow Airport, visiting other airports .
There is no obligation to pick up a bug from a cache as Geocachers can simply "discover" the bug on the website. This is where the cacher finds the travel bug and logs it as remaining in the cache it is already in and does not move it on.
Use the Golden Rule when you find a Travel Bug. Most owners would rather see their Travel Bug do a lot of travelling, so try not to hold on to a Travel Bug for too long. If you plan on holding onto the bug for more than 2 weeks, make sure to send a courtesy email to the owner letting them know.