The barest of definitions include an item that moves from geocache to geocache. This item can be a fuzzy animal, a die-cast car, or a coin. Really the possibilites of what a hitch hiker can be is endless.
As a main stream geocacher, by far the most likely hitch hiker one will encounter will be the Groundspeak-branded Travel Bugs trackable on geocaching.com. However, there are other flavors from other sites and types which may include geocoins.
A hitch hiker doesn't have to be part of any online site or trackable. The most basic form is an item with a note attached asking the finder to place them in another cache or to further some goal.
Some consider a hitch hiker a trade item, some do not. This will depend on one's ideas of "ownership" of the hitch hiker. Some consider the hitch hiker to remain the property of the person who created the hitch hiker and released it to the wild. Some consider it a trade item and therefor subject to the same rules such as transfer of ownership. On geocaching.com the hitch hiker's page will remain under the control of the person who orignated the item and some claim the proof the hitch hiker remains the property of the orginator and is not a trade item. Therefor it is not traded for like a trinket. Still others don't feel right taking something and not leaving something.
Sites That Track Hitch Hikers
There are personal sites that track personal hitch hikers, as well.