Geocaching is the sport/hobby of using those billion dollar satellites positioned round the Earth to help find small plastic boxes and containers hidden here there and everywhere!
You need a GPS system to help take you to the co-ordinates, once at the 'ground zone' you then hunt for a hidden container. The container can be a large ammo can or a tiny 1cm nano. Caches can be hidden in both urban and rural areas. Currently there are 60,000 caches hidden in the UK and nearly 1.1million around the world. Once you have found the cache then you log your find in the log book, swap 'treasures', re-hid the cache in the exact location as you found it and move on to the next one. You then record your find on the geocaching website.