We did this as an activity days group, but it would be fun with YW too!
In a nutshell, geocaching uses a GPS or GPS enabled phone to locate "treasure" that has been specifically hidden for this activity.
Where I live, the geocache trail is in a large park. We got the co-ordinates from THIS WEBSITE. You start with one set of coordinates, then you find the hidden object that contains the next set of coordinates. I believe we had to find 4 sets of coordinates before finding the treasure. There may not be a geocache in all areas.
The coordinates and treasure can be hidden in all sorts of ways. We found one set of coordinates rolled into an old film container and hung in a pine tree. One was in a water proof plastic pouch that was magnetically held on a metal box. The treasure, or cache, was in a big coffee can, hidden in a rocky outcropping.
Inside the cache were all sorts of silly objects: golf balls, a mirror, a small compass, and things like that. If you take treasure, you must put something in the container for someone else to find. There should also be a "guest book" to sign. It's fun to see where people have come from, who also found the cache!