Yep.... I've Done several geocachings.
The rules are pretty simple really. People hide containers all over the world along trails, deep in the woods, along lakes, etc. - all at or around ground level and NONE buried: the caches are filled with a small logbook, some simple "dime-store" like items and they post the location in longitude and latitude, along with a simple set of clues to help the person find the cache at the Geocaching.com website.
I have hidden several, but removed them after finding that more people were getting deer-ticks than they were finding my hidden containers - the NJ woodlands (The Pine Barrens or Pinelands) are abundant with deer-ticks which can infect you with Lymes Disease (which I got and which triggered my Spinal Mennegitis - which I still suffer complications from even today, 4 years later).
But when you find a cache, you log into the book, remove an item and replace an item in the cache - noting what you took and what you left behind. Then you log-in on the website, and make similar notation there.
The point of Geo-Caching is really to show people that "you can't always get there from here" meaning that, even with very good instructions, it can be very difficult to find the cache because (for example) you may be 200 feet away and heading right for it, then all of a sudden a stream or other obstacle is in front of you - you may have to walk a mile out of the way to find the single road or trail which takes you to the cache.
Some people are whacked and literally have THOUSANDS of finds. There are fanatical people in every hobby, but this one requires a lot of spare time to track down and document thousands of finds.
It is a fun time, a great family event and it helps everyone learn about geography and GPSing.