With a few exceptions, the history of the Marvel Universe does not diverge from reality until the advent of World War II. The so-called Golden Age witnessed the rise of the first costumed heroes: most notably Captain America, but also Namor the Sub-Mariner, the Invaders and the original Human Torch. They fought on the Allies' side against the Nazis, battling evil villains like the Red Skull and aiding Allied soldiers like Nick Fury and his Howling Commandoes.
The advent of the Atomic Age after World War II saw a rise in the radiation in Earth's atmosphere. As a result, the Marvel Universe witnessed a sharp increase in the number of mutants--men and women born with super powers instead of receiving them through scientific accidents or the like. Mutants are feared and hated for their abilities in the Marvel Universe, creating sharp political schisms between those who support mutant rights and those who want to see mutants destroyed.
The 1960s witnesses the rise of Marvel's classic characters, often created through scientific mishap. Their ranks include the Fantastic Four, the Incredible Hulk and the Amazing Spider-Man. Entrepreneur Tony Stark built a suit of battle armor--going by the name of Iron Man--and eventually formed the Avengers, a group of like-minded super heroes who battle the fiercest villains on the planet. At the same time, Professor Charles Xavier assembled a team of young mutants called the X-Men to protect normal humans from mutants who may wish to harm or destroy them.
Epic battles have marked the Marvel Universe in the later years of the 20th century. Among the most notable were the Kree-Skrull War, a battle between two alien races which draws in the Avengers; the Secret Wars, in which an omnipotent being called the Beyonder gathers Earth's greatest heroes and villains on a distant planet to see which side is stronger; and the Marvel Civil War, in which the need to identify and control costumed heroes leads to a schism in the superhero community.
Because comic book lines last for decades, official history becomes quite complicated and even contradictory at times. Marvel's superheroes remain basically the same age for many years, which is never officially noted. Furthermore, the addition of new writers and new comic book lines creates a need for new continuity rules, which need to take past events in the Marvel Universe into account. Furthermore, the Marvel Universe itself contains numerous so-called alternate universes, where heroes lost, villains won and cataclysmic events come to pass differently than in official continuity. This means that the official history of the Marvel Universe can change at any time for any reason.