Think of it as a treasure hunt. With pots/finds (caches) all around the world. Yes, even near you (maybe so close, you don't even have to get into your car). All caches try to get placed at points of interest/beautiful views.

Use of a GPS unit, guides you from the start of the hunt, to the next clue, the next (mostly just a couple) and so on. At the end, you will find the cache. Usually a Tupperware box, with items in it (great for kids), and a logbook. It is protocol if you take something, you leave something. A lot of Geocachers are in it for the scenery/views, anyhoo, and take nothing. Sign the logbook. Go home, and rest those weary legs :)

Best of all, it's free.

The caches and more information, here.

The British Association, here.

Don't know how to use a GPS unit? Try this.