Sorry Duncan. Looking back on my opening post, and Ric's follow-up, it does appear we were speaking in tongues

. The basic concept is that there are over 1 million small "caches" hidden all over the world, placed by volunteers. Most take the form of a little plastic container. Often inside the container are items such as a log book (or a sheet of paper to sign), sometimes a pen, trinkets and many other items depending on the type of cache. The idea is to navigate to these caches and locate them using your GPS.
Often the caches are in urban areas, or at least in the public view, so you must be careful not to bring the location of the cache to the attention of "muggles" or non-geocaching folk.
You need to be a member of the main site to search for caches in your area
http://www.geocaching.com/ . Some caches are maintained by local geocachers. It is a great pastime, one which I have been trying to get into for a couple of years, but other hobbies have been filling up my days (and nights).
The hobby is massive.