There's four "Ultimate" series, Spider-man, X-men, Fantastic Four and the Ultimates themselves (who are kinda the Avengers.) The point of them was to take established comic book characters, and hit "reset" so to speak with the continuity. Telling the same basic story's again, but updated, without all the baggage of 30+ years of past continuity. Let the characters be teenagers again, like they were initially.
The art and writing is typically top-notch, and the Ultimate universe is somewhat more gritty and dark than the standard one. The Ultimates especially is dark, as Mog Effloe mentioned; spouse abuse between the Wasp and her husband, a cannibalistic Hulk, a severely psychologically disturbed Betty Ross, Tony Stark's playboyism isn't played up as part of this role; it's a real problem, Nick Shield is a manipulative bastard. I disagree that Captain America is "nasty" in this series, although his use (abuse) of Bruce Banner/The Hulk wasn't all about truth, justice and the American way, by any means.
X-men is still dark, but not as much. Spider-man is probably the most traditional in terms of atmosphere of the bunch, but its still got Kingpin and the original Green Goblin doing things that the comics code would never have approved of. All in all, it's a set of series for the fans, recognizing that people who have been reading Spider-man, the X-men, etc. for all these years are now grown up and want more sophisticated stories with more sophisticated characters, but ones that are still true to their roots.
And like I said, the Fantastic Four I'm not really familiar with. They are regular series, Spider-man and X-men have run for a few years now and have a good 50-60 episodes each. The Ultimates is much more sporadic, and of course FF is new. The good news is, they are being bound as trade paperbacks just a few months after they get a good 6-7 issues put together, so you can collect them without collecting all the individual issues if you like.