This is kind of hard to explain, but a lot of times, there's nothing in there but a log (a notebook or piece of paper type log) to sign.
The idea is not to find a treasure...the idea is the hunt. Geocaches range in size from a magnetic the size of a hearing aid battery to a full-on ammo can. The biggest we have seen is the storage compartment on a retired 2 seater plane. The larger ones have "swag" in them...mostly coins, stickers, and trinkets of the sort that you will find in a vending machine. Rule of thumb is if you take something from the cache, you replace it with something else. If the cache is big enough, we usually leave a marble. Sometimes there are "trackables" that have a unique number assigned to them. You can literally track where they have been by looking up their number on the website. We brought a trackable back from Hawaii and left it in Delaware, and it's now in England.
You find the cache using GPS coordinates. When you find it, you sign the log, and you replace it where it was hidden. You then go to the website and "log" your find on the individual cache page (each cache has its own page).
It has been a way to get off our asses and go out and wander around. It's great on road trips because there are caches at most rest areas across the country. If you give me a town name near you, I can tell you how many caches are in that town. There are a couple hundred in Ames.
It's a fun activity with kids/grandkids, too.