official website provides more information about this interesting game.
You basically create an account on the website and then search for treasures (or caches) that are hidden around your area.
Each cache is identified by its GPS coordinates. You punch the coordinates in your GPS device and let it guide you to the cache location. Since there is always a margin of error, the GPS will basically take you very close to your target, and then it is up to your skills to actually locate the cache.
The game is very popular in U.S. However, there is not much interest in the game here in Kuwait (where I live). When I created my account and started searching for hidden caches, I could only find a handful hidden caches in Kuwait, and most of them were located on restricted U.S. military areas.
One cache did catch my attention. It was hidden in Jal Al-Zour on top of Mutla Ridge (considered to be the highest point in Kuwait). The ridge is a long hilly area made up of sand and stone that lines up the bay of Kuwait. Since it's a family oriented game, all of us got into our 4x4 and decided to go on an adventure to find the treasure.
What has geocaching got to do with photography, you say? Well, it is an excellent opportunity to go out and explore - which in turn gives you an opportunity to take pictures.
We did manage to locate the cache - and take some nice pictures in the process.