http://xbox.ign.com/articles/628/628863p1.html
As far as structure is concerned, Ultimate Spider-Man splits between various story-driven missions detailing the exploits of the Ultimate Universe version of Spider-Man and Venom (for more on that, just visit IGN Comics). You play as both characters, only one at a time, in an intricately woven tale written by famed comic scribe Brian Michael Bendis. Story missions are tied by freeform sections where you're free to swing about Manhattan battling evildoers at will. Whenever a citizen is in distress, a beacon will appear on a 3D map found on the lower right-hand of the screen.
Of course, you're not required to do anything about it if you don't want to. If you just want to swing from skyscraper to skyscraper all day you can do that too. Manhattan won't be as large as it was in Spider-Man 2, but it there'll be more stuff to experience, such as famous Marvel landmarks like the Baxter Building and Empire State University. In a sense, Ultimate Spider-Man plays like Spider-Man 2, minus the fat. Plus, the storyline takes you into Queens this time around, so there's even more to explore.