Wikipedia have full details of the sport (or hobby) here. and part of their page states:
For the traditional geocache, a geocacher will place a waterproof container containing a log book (with pen or pencil) and trade items then record the cache's coordinates. These coordinates, along with other details of the location, are posted on a listing site (see list of some sites below). Other geocachers obtain the coordinates from that listing site and seek out the cache using their GPS handheld receivers. The finding geocachers record their exploits in the logbook and online, but then must return the cache to the same coordinates so that other geocachers may find it. Geocachers are free to take objects (except the logbook, pencil, or stamp) from the cache in exchange for leaving something of similar or higher value.
A Smokey Bear Geocoin.Typical cache "treasures" are not high in monetary value but may hold personal value to the finder. Aside from the logbook, common cache contents are unusual coins or currency, small toys, ornamental buttons, CDs, or books. Also common are objects that are moved from cache to cache called "hitchhikers", such as Travel Bugs or Geocoins, whose travels may be logged and followed online. Cachers who initially place a Travel Bug or Geocoins often assign specific goals for their trackable items. Examples of goals are to be placed in a certain cache a long distance from home, or to travel to a certain country, or to travel faster and farther than other hitchhikers in a race. Higher value items are occasionally included in geocaches as a reward for the First to Find (called "FTF"), or in locations which are harder to reach. Dangerous or illegal items, weapons, food and pornography are generally not allowed and are specifically against the rules of most geocache listing sites.
A Travel Bug from Hong Kong attached to a Common Stored Value Ticket.A major site to join is called, simply, Geocaching.
If a geocache has been vandalized or stolen it is said to have been "muggled". The former term plays off the fact that those not familiar with geocaching are called muggles, a term borrowed from the Harry Potter series of books which was rising in popularity at the same time Geocaching got its start.
Although I have been a member of the Geocaching site since 2007 I have never actually got around to getting involved. This year I have a seven year itch - for Geocaching not for my marriage! so I intend to finally get involved.
Have you ever indulged in this hobby? Did you get involved? Were you successful? Did you give it up? Please, under comments, let me know your views on this fascinating hobby.