People hide things in containers, from eraser sized containers to ammo cans or pickle buckets in size. They have a logbook in them and some times trinkets (usually for the kids). People hide it (never buried), and post a webpage about it. You download the coords to GPS, and the description to a PDA, and search for it. When you find it, you sign the log, put it back where it was hidden, and also log it on the hobby's web site. People have t-shirts that say " I use billions of dollars of technology to find Tupperware in the woods".
It gets you out all over the place. Over five years ago I wanted to get some exercise, but thought walking or biking up the same trail was bring. I saw an article on it, tried it and was hooked. It's better than being a couch potato all weekend.
There are hundreds of thousands all over the world. Some soldiers even geocache on the military bases in Iraq. There are likely several less than a mile from you, unless you live way in the sticks ( Alaska has about 1000 total). A couple met at a picnic for the hobby in my area and their daughter (now three) has about 1600 finds (when she is with mommy or daddy, they write a cute log for her). Families and all ages of people can do it.
I have gotten to see just about all the parks in my area, and many beautiful views. I have to travel for work, so I download several where I am going to be and if I have time, I look for a few after I am done for the day.
In 5.5 years, I have found about 4000 caches.