What exactly is it and is it worth using? I've been successful in filtering jerks who try to spam me using the regular mail functions. How does the Spam Assassin work?

It works very much like a regular email filter. You can check out the details here:

http://www.cpanel.net/docs/cp/spamAssassin.htm

That shows you how to install it and includes a few links to still more information. I personally don't use it, and it you're already filtering out most of the junk mail you get anyway, it probably wouldn't be of much benefit to you.