Geocaching can be loosely described as a modern-day treasure hunt. Using GPS units, regular people (often nerds) go out hunting for caches (the treasure) hidden by other regular people.
The caches range in size from smaller than a dime to refrigerator-sized (and larger!). There is always a place to sign your name to mark that you were there! (X marks the spot.) Sometimes there are rewards for finding the spot; sometimes the prize is just leaving your mark.
Geocaches can be found just about anywhere. Rumor has it that there is even a cache in Mongolia.