Started in 2000 as a way to to reboot and enliven classic characters, the Ultimate imprint of Marvel comics takes place in its own universe. Characters in the Ultimate Universe are often physically younger than their Marvel Universe counterparts, and events tend to unfold in radically different ways. For instance, characters long-dead might still alive in the Ultimate Universe, while others may not exist at all. Plot-wise, memorable events happen in different ways with vastly different outcomes.
Ultimate Spider-Man, for instance, currently stars the first mixed-race version of Spider-Man, Miles Morales, who assumed the Spider-Man mantle after Peter Parker died. As previously mentioned, if we go by the multiverse Official Reality Number scheme, the Ultimate Universe is referred to as Earth-1610—but we're just going to call it the Ultimate Universe to keep things simple.
From an editorial standpoint, the Ultimate Universe has traditionally been a place for Marvel to test out new versions of characters for both the Cinematic Universe or the regular Marvel Universe. For instance, the Ultimate version of S.H.I.E.L.D. director Nick Fury was based on Samuel L. Jackson, while the original (Marvel Universe) Nick Fury was a white guy with a mullet. (Both have an eye-patch.) Eventually the Sam Jackson Fury made his way into both Marvel Cinematic Universe and, through some zany storytelling hoops, the traditional Marvel Universe.