According to wikipedia, Geocaching is an outdoor recreational activity, first played in May 2000, in which the participants use a Global Positioning System(GPS) receiver or mobile device and other navigational techniques to hide and seek containers, called "geocaches" or "caches", anywhere in the world.
What is a geocache (geo-cash)?
A typical cache is a small waterproof container containing a logbook where the geocacher enters the date they found it and signs it with their established code name. Larger containers such as plastic storage containers (Tupperware or similar) or ammunition boxes can also contain items for trading, usually toys or trinkets of little value.
For the traditional geocache, a geocacher will place a waterproof container containing a log book (with pen or pencil) and trade items then record the cache's coordinates. These coordinates, along with other details of the location, are posted on http://nagpurcache.blogspot.com. Other geocachers obtain the coordinates from that listing site and seek out the cache using their GPS handheld receivers. The finding geocachers record their exploits in the logbook and online. Geocachers are free to take objects (except the logbook, pencil, or stamp) from the cache in exchange for leaving something of similar or higher value.
Container sizes range from "nanos", which can be smaller than the tip of finger and only have enough room to store the log sheet or even larger containers. The most common cache containers in rural areas are lunch-box sized plastic storage containers.