My impression was that the geocaching thing assumes that you'll use GPS. Perhaps it's grown to include other modes of search.
There seem to be a number of organizations active, although I haven't kept notes on sites I've stumbled across.
Although caches in the great outdoors are probably most numerous, there are some in urban areas, and I believe I saw one site that marked "handicap accessible" as one of the characteristics people could cite for their caches.
Some of the GPS makers have bits of info on the hobby at their websites. (And some people appear to consider it "a bit more than hobby" - more like a life-style.)