Geocaching is a relatively new outdoor sport: a search for (typically) small, waterproof containers full of "treasures". What makes the search different from an Easter-egg hunt is that the containers, called "caches", are found using the Global Positioning System ("GPS"). Caches are placed anywhere in the world and their exact geographic coordinates are published on the Internet. Other participants obtain the coordinates and other information from the Internet and, using handheld GPS receivers, seek out the caches, hoping to write of their exploits in a logbook contained therein. Geocachers are free to take objects from the cache, in exchange for leaving something in return, so there's always a "treasure" for the next person to find.
Typical cache treasures aren't especially high in intrinsic value, but often make interesting souvenirs: $2 bills or other unusual coins or currency, kids' toys from fast-food restaurants, ornamental buttons.
There are several variations on the basic geocaching theme. Some caches require the seeker to solve a puzzle or visit one or more intermediate points to determine the coordinates of the actual cache. Some are "microcaches", too small to hold a logbook. A very few are virtual caches, with no container at all, usually at a site which is interesting in and of itself.
Geocaching is growing rapidly in popularity all over the world.