My folks and friends are obsessed with it. I have avoided it so far as have other things to be getting in with. But I know more about it than I should.
Boxes or caches cannot be placed on any private land without the land owners permission. If you do it will not be published. If you wing it and you are found out you're account is destroyed. Part of the reason I have avoided it, so many rules. However it is quite fun. I used to letterbox and it's similar. My brother makes really elaborate caches. It's well worth it. It's not too Clicky round here and they hold annual events in the UK. Where everyone meets up and go caching, and show each other their coins. I had some made for my folks last Christmas. On the geocaching German coin shop. Supplied them with photos and they laser etched the pictures on. Brilliant quality.
The next meet is up north somewhere. It's usually a camping affair. Although the folks tent is a 3 double bedroom thing with carpets and heating - glamping apparently.
Caches come in (from memory) 4 sizes. A cache, a micro, a mini and a nano. Each one smaller than the other but also harder to find. A set of GPS co ordinates are obtained when The cache is placed. You use these to find it. Easier than it sounds. Some custome made hideyholes are things like a magnetic snail shell. Or a sprig of fake grass.
A GPS is essential. No doubt some on her do it and will refute this but the alternative is a car satnav or iPhone app for geocaching. Neither is accurate enough and judging by the vitriol that I hear about such people using only these devices to place the cache I would invest in a GPS. If you use these to place out a cache, and no one can find it and complaints arise, yOur cache will be removed from the database.
It's a world wide thing and there are moves to get one on the moon. There is one in space. And there are a few only reachable using a deep sea submersible. It's a fun sport and believe me it's a sport (I've helped them find a few on Coastal paths) , mums nearly broken her hip, been run over on the moors and my brother has walked into mines on dartmoor. Quarter of a mike underground walking across flooded shafts and up to his waist in water to reach these things. I stood guard recently when he went into a cave and rummaged around in the rubble on his belly. So it can be as easy as a gentle walk from the car on a motorway lay-by to using a boat to reach remote hidden sea caves (my brothers next set to get In his kyak) hope you enjoy PH.
They have a Facebook page and a couple of forums.