Caches may be Gallon size Plastic Jugs (Glass is not used for safety reasons), Old Army Ammo Cans. Some are as small as a magnetic key keeper box. Some that are "micro" may be hidden in a 35mm film canister. I have seen one hidden in a hollow 3/4 inch bolt magnetically stuck to the bottom of a park bench to look like the hardware on the bench.
They may contain a simple log and pen so you can enter the date and time and your name when you found it. Whether you took a trinket, left one, or neither. The micros require naturally, that you bring your own pen and sign the small log.
Contents:Some contain trinkets such as commemorative coins, Rubber Balls, souvenirs, Sometimes small amounts of money. All manner of things... glow sticks, mini flashlights, deck of cards. etc. Generally if you take something, you try to leave a little something. So I carry a plastic baggie in my vehicle with Ink pens, glow sticks, balls, some quarters.
Travel Bugs: These are special "Coin Bugs" that have a serial number. If you find one you take it log it and if you travel allot drop it off at another cache you find. Enter the serial number in the datatbase. You can track these "Bugs" all over the world as they move around.
Equipment: A simple GPS, and some use GPS enabled phones even. I use a Magellan Gold Series GPS that goes for about $100 on ebay.
Places: I have found some in magnetic key boxes attached to the underside of a counter top in a phone booth. One under the small base of a flag pole in a park. Some are in busy places and you must be careful not to be seen getting in and out of the cache
Enter the coordinates that you get from the database, head out to the location using the GPS to find it, and then try to pinpoint the location. When you get down to the last meter or two you will have to look around and use your imagination as if you had hidden it.
It is fun, difficulty depends on what you want to do by your selection of the cache to be found.Try it, I think you might find it fun and enjoyable. You might even want to hide your own and list it.
Learn more at Geo Cache