Usually just trinkets and stuff. The general idea is that you take one thing and leave one thing, often something from a previous cache that you visited, so stuff moves around. Also, there's a logbook to sign and date.
its pretty tight actually, I do it w/ some of my cousins and chit when I visit them, the lil kids like them mostly cause you get to hunt around and find fun shtuff, great excuse to get outside and bike and hike around!
I do it. I have a garmin GPS and it is some good fun and I take my nephew cachin' with me. Ya just go on the website, get the coordinates and go hunt it down. You can find all sorts of stuff in it and sometimes a camera. Thn ya hide it in a different spot an' post up the coordinates. Oh, and if ya take somethin', ya gotta put somethin' else in. It's one hell of a good time, or just a time killer if yer bored.
There's a part of geocaching called travel bugs. They are toys or whatever that you get a dog tag with a serial number that you attach to it. It goes from cache to cache with pics taken of it and stories told of it's adventures along the way.
There is another part called a reverse cache. It is where you are told of a specific object to find and when you find it you take a picture of it in the backround with you and your GPS in the photo. They have to be unique so a bunch of people can't just go to the same one and claim it as a find.
Way back when Geocaching started there was a reverse cache called Yellow Jeep. If you came accross one you took your picture and posted the coordinants on the site. This became one of the most popular reverse caches for people to find.
Jeep found out about this and a couple of years ago they became a major sponsor of the Geocaching site. They gave away 100,000 Yellow Jeep Travel Bugs and if you happened to have a certain one on a certain day your could win a free Jeep or other prizes.
Like Jeeps or not I think it was a great marketing tool and it helped the Geocaching site get to where it is today. I wish I would give myself more time to go and do it again since it is a great way to get exercize, take the dog for a walk or if you have kids for an adventure. Best of all it's a pretty cheap form of entertainment. You will learn a lot of local history you probably never knew about, find great places you would likely never see otherwise and make new friends as you meet them on the trail.