Hi Togley,
It's basically a global treasure hunt using your hand held GPS. People hide small caches and publish the lat/long co-ordinates to their location on a main website. There are often puzzles to solve to find the exact spot either on your journey to it, or at home on the web.
Caches tend to fall into three catagories - kiddy friendly in local parks or countryside, fiendish puzzles or those in more extreme locations.
If you have a GPS just google geocache, the whole thing is explained on the main site.