What is geocaching?

"Geocaching is an outdoor treasure-hunting game in which the participants use a Global Positioning System (GPS) receiver or other navigational techniques to hide and seek containers (called "geocaches" or "caches") anywhere in the world."
Source and further information:
http://en.wikipedia.org/wiki/Geocaching

Further information here:
http://factsfacts.com/geocacher.htm
http://geolex.locusprime.net/
www.geocaching.com/
http://www.brillig.com/geocaching/

Geocaching is an adventure game for gps users. The basic idea is to have individuals and organizations set up caches all over the world and share the locations of these caches on the internet. GPS users can then use the location coordinates to find the caches. Once found, a cache may provide the visitor with a wide variety of rewards.

Take a look here : http://www.geocaching.com/faq/

where to go for info is geocaching.com

Geo-caching is a prectice similar to letter-boxing except that in geo-caching you are given coordinates you need to find using GPS. In letterboxing you are given clues and directions but no coordinates. At the coordinates there will be a cache, sometimes there are rubber stamps and notebooks which you can use to log your finds and indicate your arrival in the cahe's logbook. There may be trinkets which you can take or a note for those who find the cache. Anyone can set up a cache and put the coordinates online. Orienteering is also similar but there aren't caches, but points that need to be found using mostly terrain recognition. It is more of a sport than the others but geocaching could be set in a sport environment too. Letterboxing is quite leisure.

http://www.geocaching.com/
http://www.letterboxing.org/
http://www.us.orienteering.org/

Without looking it up, I would say it is storing dirt, but a more accurate and technical answer can be found here: www.geocaching.com/