Manga - Help!

I have the opportunity to (possibly) review graphic novels so I'm trying to educate myself on the subject. I have a friend who's given me a reading list for comics (mostly of the super-hero variety), but I realize I know absolutely jack about manga. As this is the biggest thing in comics today, anyone have suggestions as to where to start? What series? What are the sub-genres? Are there any truly groundbreaking books?

So far all I've read is "Oh My Goddess!" which I admit was fun. You do get a lot more stares on the train when you're reading comics, though. I caught one guy looking at my book, and when I told him it was manga, I don't think he had a clue what a meant.