Pretty much any GPSr will do. If the unit doesn't have a moving map, it helps to bring along an accurate map, preferably a topo. I have an eTrex Legend, and MapSource Topo, and I download the topo maps for the area into it. WAAS helps, but only if the person who placed the cache also has WAAS.
There are usually toys or trinkets inside the cache. You're supposed to take one, an leave one of your own. There are also markers with a code on them, you're supposed to take it out, enter the number in a web site, and then put it in the next cache you come to. You can then track these markers on the web to see where they've been.
EDIT: Any more questions, go here:
http://www.geocaching.com/faq/