The Ultimate Universe is not to be confused with the “Heroes Reborn” universe (although a nifty explanation for the Ultiverse was provided in Paradise X #9 or 10, I believe.
The Ultimate series are basically what you said: Marvel hit a reset button. It’s the comics redone for the 21st century, much as John Byrne did with Superman in 1986. The thematic elements are there (Peter Parker is bitten by a spider, gets powers, Uncle Ben is shot, yadda yadda), but things are rewritten to “fit” our world better. (Think of the Ultiverse the way you compare the Batman cartoons to the Batman comics: everything you understand is in both, but they’re portrayed differently.)
IMHO, Ultimate Spider-Man is a fabulous series. It’s no Lee/Ditko (or Lee/Romita, for that matter), but it’s a good, clean introduction to the character. And there’s lovely hardcover and paperback collections of the series if you’re looking for the full story.
Ultimate X-Men scares me, because it’s rather unfamiliar compared to the X-Men I grew up with. They’re more militant, separatist, and a little punk. I fear that a lot of UXM will become dated very quickly–Ultimate Spidey at least feels like it could be readable for a long time without another reboot in 20 years.
The Ultimates is what I believe Fenris once described as like a trainwreck: horrific, but you don’t want to look away. It’s not The Avengers. You want the Avengers, read the Avengers. Here, Cap is a military man with a pole up his arse, Thor is some environmentalist wacko, and the Wasp is asian and frequently naked. (I don’t mind the Asian part–they made Nick Fury black, and nobody noticed–but the cheesecake appeal is overdone.) I’m also noticing it borrows from movies a lot: one issue read like The Matrix while another reeked of Independence Day. And it’s supposed to be original. Yipee!
Ultimate Fantastic Four is coming. I don’t know a lot about it, except 1) there’s no rocket ship involved, and 2) Doctor Doom’s name isn’t Doctor Doom.
There was an Ultimate Daredevil/Elektra mini-series. DD has appeared on occassion in the Ultiverse, although they’ve kind of hinted that their DD isn’t really that different from “our” DD.