Derek,
Basically they decided to bring a fresh, "modern" spin on all the top name Marvel Series'. They decided to start over with a brand new universe. So with the hiring of Joe Quesada as Editor-n-Chief of Marvel about 5 or so years ago, they came out with the "Ultimate" Universe. The first on-going series was Spiderman. The impressive thing about Ultimate Spiderman is its had its original writer (Brian Michael Bendis) and artist (Mark Bagley) for its entire run since its inception and they're still going strong, and that series is in its 70s. Something very rare nowadays. Ultimate X-Men was the second series to come out. Mark Millar and Adam Kubert started that series but both have since left and there has a been a slew of writers (mostly good) and artists (some great, some very very BAD) on that series. Lots of new series and mini-series would soon follow (The Ultimates-Avengers, Ultimate Fantastic Four, Ultimate War, Ultimate Daredevil/Elektra).
The only series I've collected from the Ultimate line are X-Men (all of it), FF (first story) and the Ultimates before it was cancelled, its since been restarted but I haven't gone back. The two mini-series I have is Ultimate War and the Daredevil one.
X-Men is awesome, the first 12 issues especially., the fresh spin on all of the mutants is always interesting, consistency is its problem.
War was an Avengers/X-Men fight Magneto cross-over.
Daredevil was good but sappy, more in the way of relationships instead of ass-kicking.
FF was not very impressive.
And the Ultimates was great just to behold the beauty that is Bryan Hitch's work. Although I've never bought any, I always hear great things about Spiderman and what I've read has been pretty good.