It is a GPS game of hide and seek or treasure hunting. People all over the world hide these containers in rural and urban areas that contain some cool items or swag. There is a good chance there is a cache not even a mile from you. There is an App for the Ipod touch/phone and one for android phones. With the android phones there is a second free app that is just awesome. I use a combination of a Oregon 450 and my Droid X with an app called c:geo (the free one). You will reference a site called geocaching.com. From there you will create a free account if you choose and you will be able to see the caches when you do a search.
I found one purely by accident one day, I thought about emptying the contents and taking a dump in it, but my conscience got the better of me, and instead I put a $20 bill in it and moved it about 50 feet from where it was at, I?ll see if it is still there in a few months