The process of burying a capsule filled with a logbook and various other items, recording the location with a GPS receiver and then publishing that location on the web. Other people can then find that location using their own GPS receivers and make a note of their find in the logbook. They can also take one of the items in the capsule and leave one of their own. Its kinda a new tech adventure game/treasure hunt.
The first geocache was set on May 3, 2000 outside of Portland, Oregon at
N 45°17.459'
W 122°24.800'
As of this date, there are caches in 20 US states and 11 different countries.
More information, including a database of currently known geocaches can be found at http://www.geocaching.com