Gosh, this one got going while I wasn't looking.
It's a kind of electronic treasure hunt with the information about the "treasure" on the internet, which you load to your GPS and then go hunt!! El Jefe is absolutely right in his description but to clarify what is hidden in the cache, it's usually something low value to make it worthwhile for children to find ie the sort of plastic toy you would get in a Christmas cracker or in a goody bag, sometimes there are keyrings, erasers, etc. You take out something you like and you replace it with something else. That's for the children anyway, adults are just pleased that they found the cache and were able to sign the log. The cache itself is usually no bigger than an ice cream tub and often as small as a film canister. The minimum they contain is a notebook and pencil for visitors to log their "find". However, many people like to place a "travel bug", which may be a specially minted coin or something of their choosing, which has attached to it a purpose made dog tag with a special tracking number. These items are collected from one cache, and placed in another and travel around that way (they are also known as "hitchhikers".) Each time someone finds and moves one, they log it on the main geocaching website.
As an example, I have sent a small teddy bear keyring, by the name of Isaac the Bear, off on his travels which are logged on a map on the geocaching website). When despatching said bear, I was able to post a photo of him and express where I wanted him to go. I just wanted him to see the world, and if possible, to be photographed with children he spent time with or at interesting places. Since I "released him" in 2008 he's been all over the Czech Republic via Germany and is currently back in Germany. Amongst others, there is a photo of him with the German high speed "ICE" train, and also sitting on top of someone's television during the Royal Wedding.
The point of the little film I showed you is to illustrate the purpose of the website it was posted on ie that sometimes these travel bugs get "stuck", ie they are not moved on from a cache. The idea of that website is for people to log details of a stranded travel bug so that someone local to its hiding place can either check that it is still there and/or move it along a bit.
There are millions of caches worldwide, and there's bound to be one near you! Further details at
www.geocaching.com
