What is Ultimate Marvel?
The simplest way to think about it is as a new-millennium, ground-up reboot of the four most enduring Marvel Silver Age properties – Spider-Man, X-Men, The Avengers (as Ultimates), and Fantastic Four. That frequently made for a take on Marvel's heroes that was both more edgy and more grounded. There were fewer aliens, magic, and resurrections, and more real-world socio-political ramifications. While the line wasn't for Mature Readers, the combination of those factors could sometimes lead to a grimmer take than the main Marvel universe.
The line began in 2000, and during its near 17 year history, it saw a number of re-toolings and writers taking the line in different directions. Finally, it was decided to end the line in as part of the larger Marvel-wide streamlining that was brought about by 2015's Secret Wars crossover event.