A Travel Bug (TB) is one of those aluminium tags attached to something that you find in a cache. They are items that you can remove from a cache without needing to swap with anything else. When some people are learning how to geocache they aren't always sure of what is the right etiquette when it comes to handling TBs.
The idea of TBs is that geocachers move them to another cache so that the TBs have little adventures of their own. TBs can log tens of thousands of miles as they travel around the world.
Every time someone retrieves or places a TB its owner receives an email and can track its progress. In this way they can enjoy its travels vicariously. The geocaching site has a feature where TB voyages are plotted on Google Earth. It's amazing to see how far and wide these things travel - just by hitch-hiking!
So what do you do if you find one? What is the correct etiquette?
Firstly realize that they belong to someone else and while it's in your possession it's your responsibility to look after it.
You should place it in another cache as soon as possible. Don't leave it for many weeks before you place it somewhere else. The only exception I would make to this is if you are planning to go on an overseas trip and the goal of the TB is to travel around the world.
The cache in which you place it should be one that is visited fairly frequently. In other words don't hide it in a cache that is rarely visited, such as a difficult puzzle cache, or one in a remote location etc.
If you come across a TB in a cache that is visited infrequently, do the person a favour and move it on. They'll be very grateful.
If you own a TB, please don't attach some large object to it. It can be very difficult to find a cache large enough to accommodate something large. If I find such a TB, I usually won't move it on for that reason.
If you do lose a TB that is in your possession, inform the TB owner. I'd rather know than have to chase you up. One of my TBs didn't move for months. I kept following up with the last person who eventually placed it somewhere. It took 6 months though.