This Google post lists the first ever documented geocache back in May, 2000 where someone named Dave wrote:
Well, I did it, created the first stash hunt stash and here are the coordinates: N 45 17.460 W122 24.800 Lots of goodies for the finders. Look for a black plastic bucket buried most of the way in the ground. Take some stuff, leave some stuff! Record it all in the log book. Have Fun! Stash contians: Delorme Topo USA software, videos, books, food, money, and a slingshot!
Well that's quite a stash! Don't expect to find stuff like that today, at least not typically. A typical cache is usually a small waterproof container - the old plastic film containers are often used - and inside you will usually find a logbook and sometimes a small trinket. Finders of the cache log their code name or real name into the logbook along with the date they found the cache. You can also take the trinket and replace it with one of similar value. In this way some trinkets find there way around the world!
Geocaching has grown in popularity as people who enjoy hiking combine that with the treasure hunting aspect of geocaching. The location of caches is recorded on various websites the most popular of which include:
Geocaching.com - this is the largest site and it claims to have millions of caches listed with members in over 200 countries
Opencaching Network - this is free site with a large set of listings. You can find it at www.opencaching.X
X is country specific and includes the following choices: .us .de .se. pi. c. lv. no. jp .uk
Opencache.com - this is a different site the those associated with the Opencaching Network. This site is free and open. Caches are approved via a community process and geocache coordinates can be obtained without having to set up an account.
You can go to one of these sites and look up where geocaches are in your city. I, for example, live in a small city of 50,000 in Ontario, Canada and it's amazing how many caches are available to find in my city. Some are less than half a mile from where I live and no, I didn't put them there!