A geocache is basically a container, hidden away somewhere, that contains at minimum a log page to sign. Often caches also contain things like small gifts that you can take and leave, disposable cameras to take pictures of yourself, and so on.
There are thousands of these worldwide (13 in Hyde Park in central London alone), and you use a GPS device to locate them. First, you go to a site like Geocaching.com, find a cache near you that you want to get to, stick the latitude and longitude into your GPS device and then go out searching.
GPS itself is not totally accurate. My device tends to get to within 30 feet of a location before you can no longer trust it. Which actually adds to the fun, since once you get to the place where the cache is supposed to be you still need to figure out exactly where it is. So far, we’ve found one under a log in the woods near us at Runnymede, one magnetically attached to a railing in the middle of Oxford, and one hidden under a rock next to a slug just off a footpath on Chobham Common.
So, it’s a geeks activity on the surface, but really for us it’s a great way to get out and hike with a purpose.