Over the years, the Marvel Multiverse has expanded from one reality into hundreds, if not thousands more mirror universes. With each new story delving into a new alternate reality, be it the familiar What If...? storyline or something entirely different, the creators over at Marvel Comics establish an entirely new world. As each new universe is created, it is given a number in the vein of Earth-616, Earth-12123, etc...
There have been hundreds of these Earths listed in detail throughout Marvel's history (many of them just dedicated to the X-Men), but only a small number could be considered to be primary timelines within the overall Marvel Universe. These typically feature a Marvel imprint or a universe that is so important, it plays a major role in the development of a new book or established series. Here are all of the main Marvel Universe timelines listed with their first appearances (or mentions), who lives or comes from them, and why they are important.
Earth-199999 (The Marvel Cinematic Universe, MCU)
Inhabitants: The cinematic versions of the Avengers, Guardians of the Galaxy, Agents of S.H.I.E.L.D., the Defenders, and all of their associated characters presented in live-action in film and television.
First Appearance: Iron Man (2008)
The MCU was launched with the first Iron Man film in 2008 and includes more than 20+ films and six television series. This world includes the street-level heroes like Daredevil all the way up to cosmic beings like Thor and Thanos.
Earth-555326 (The Animated Next Avengers Universe)
Inhabitants: The Next Avengers
First Appearance: Next Avengers: Heroes of Tomorrow (2008)
This world appeared in the direct-to-video film, Next Avengers: Heroes of Tomorrow (2008), which featured a more cartoony version of the characters for a younger audience.
Earth-3488 (The Animated Ultimate Universe)
Inhabitants: The Avengers from the Ultimate (animated) Universe
First Appearance: Ultimate Avengers (2006)
This world differs slightly from the printed Ultimate Universe and features many of the characters from the comics, but different enough to set this world apart from the standard Ultimate Universe. In addition to Ultimate Avengers, there was a sequel, Ultimate Avengers II released the same year.
Earth-1218 (This Is Our Reality)
Inhabitants: You and pretty much everyone else alive in the world at this moment...
First Appearance: New Exiles #3 (2008)
This Earth designation is the one from Marvel Comics that is our own reality. Anyone reading a comic or going through this list right now is living in Earth-1218.