Geocaching is described nicely on the http://www.geocaching.com/ website:
Geocaching is a high-tech treasure hunting game played throughout the world by adventure seekers equipped with GPS devices. The basic idea is to locate hidden containers, called geocaches, outdoors and then share your experiences online.
I was surprised by the number of geocaches very close to my home, and in fact throughout New Zealand and the World (over 1 million!!!). Type your address into the search box on the http://www.geocaching.com/ website to see how many are near you.
Inside a geocache you will find a log book to log that you have been there and depending on the size of the cache a pencil (for the logbook), items that you can swap out and replace with an item of equal value, a short overview of what geocaching is, and trackable items that tend to have a task associated with them that you can take out of a cache and help them on their journey. Logging of finds, fail to find and the movement of trackables all occurs on the http://www.geocaching.com/ website.
One of the most interesting things about geocaching is finding beautiful areas you would never have otherwise gone to, and getting some exercise along the way.
<Cross-posted on http://gianouts.blogspot.com>