From the official site: http://www.geocaching.com
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. Geocaching is enjoyed by people from all age groups, with a strong sense of community and support for the environment.
Basically its a high tech form of a scavenger hunt. I can't wait to give it a try and see what interesting places and things I can find. I'm GeocacheGeek over on the site if you want to look me up. I'll also be blogging separately on http://geocachegeek.blogspot.com and http://twitter.com/geocachegeek if you want to follow me.