The short summary version of geocaching. . . people all over hide small/med/large containers called caches and post the longitude and latitude cordianance and a short desprition of the cache on a website
Geocaching - The Official Global GPS Cache Hunt Site Sign up as a user on the website and get the coordiance of the caches near where you are or are going. Put the cordiance in your handheld GPS and go find the cache. After you find the cache you tell about your adventure on the website.
The caches can be really really big, rubbermaid containers, amno cans, coffee jars, or so very small, a pill bottle or a film canister. The small caches only contain a log for you to date and sign. The large ones contain the log and possible trade items, usually key chains, stickers, happy meal toys. If you take something you have to leave something.
Caches are placed just about anywhere you can think of. Lots of people place them at their favorite spots, nice views, places to eat or at places that have a tibit of history associated with it. And they are everywhere, my zip code has over a thousand caches.