note. ipad OS 3.2.1 can not be jailbroken at this time. only 3.2 can using the "spirit" jailbreak method. it's pretty simple. plug in your ipad, quit itunes if running, run the spirit app. it will make a connection to your ipad, run for a few seconds, then quit. your ipad will do some magic, then will restart, and you'll see a Cydia application. that's all that's needed.
if you ever decide to remove the jailbreak you simply need to do a full restore to your ipad. you won't have cydia any more nor any of your jailbreak apps.
the cydia app store is a little "wild west". they do try to keep some control on it (and have removed some rogue apps in the past) but pretty much anything goes. you will find pornographic material, apps that violate many of apple's policies (such as Firewall IP), and some low-quality junk. much of the offerings are free but some do charge a fee (one-time or subscription, depending on the app). most of the content is specific for a 3.x iphone, but there are more and more ipad compatible apps and things are getting updated for 4.x as well. there is no way to jailbreak an iphone 4 at this time.
cydia is the basic store app, but there are alternatives you can install as well such as the "Rock Installer" which is a little faster and little better organized.
just be warned there isn't as much hand-holding in jail-break apps as there are from the apple store, it's not hard to use, just that you need to be a little more aware of what you are doing.
if you want to get started with the "dark side" i'd suggest hanging out on
http://modmyi.com/ it is one of the premier web sites for that side of things.