The Multiverse Map Of the Arrowverse

EARTH-1

Earth-1 is the default Earth on which The Flash, Arrow, and DC's Legends of Tomorrow take place.

There are other series which take place on that world as well: the animated Vixen, for instance, and the defunct Constantine, as evidenced by those characters' in-continuity appearances on Arrow.

It's not yet clear whether Freedom Fighters: The Ray will take place on Earth-1 or not: in the comics, the Freedom Fighters come from Earth-X, a world where the Nazis won World War II and superheroes are literal freedom fighters against the authoritarian regime.

EARTH-2

Fans of The Flash have already seen Earth-2 up close and personal in the episodes "Welcome to Earth-2" and "Escape From Earth-2" during The Flash's second season.

The world has a kind of retro look to its technology, which is actually on par with the Earth-1 tech, whether or not it looks that way.

Earth-2 seems to be a rough corollary to Earth-3 from the comic books (or alternately, the dimension referred to as "Earth-2" during the post-Crisis on Infinite Earths/pre-Infinite Crisis era of DC's publishing), in that it's stocked with evil versions of familiar superheroes. The speedster there was Zoom, who used his super speed to enslave the world, while Cisco Ramon was Reverb, Ronnie Raymond was Deathstorm, and Caitlin Snow is Killer Frost.

The Earth-2 Laurel Lance, Black Siren, appears to be impersonating her Earth-1 counterpart in upcoming episodes of Arrow.

EARTH-3

Earth-3 is the home to Jay Garrick, an older version of The Flash whose only known adversary is the Earth-3 version of The Prankster.

1990s The Flash star John Wesley Shipp told ComicBook.com that he feels like his version of Jay Garrick had a career that was similar to his version of Barry Allen, so it's safe to assume at least for your own headcanon that the Earth-3 version of The Flash had some adventures not unlike the ones you could see on that show back in 1990 and 1991.

"I figured Jay is my version of Barry, 25 years later, essentially. So I went back and I watched a couple of episodes of the 1990/91 version to kind of remind myself what I did," Shipp told ComicBook.com. "[Jay] is much more reminiscent of my Barry Allen from 25 years ago than my Henry Allen. I went back and I was amazed how much attitude my Barry Allen had in some situations. I went back and I picked up that thread and I brought it forward 25 years, and tried to weave it in. I think that's fun for the audience, too -- that they will see elements of my Barry Allen in my Jay Garrick."

EARTH-19

Little is known yet about the world from which H.R. Wells hails -- except that it's apparently forbidden to travel the multiverse there, and there's a superhero named Gypsy, who will soon make the trek to Earth-1 to look for Wells.

Here's how DC describes this Earth on the comics side, which is home to the Gotham By Gaslight universe:

Earth-19 may appear to lag a century behind other worlds in the Multiverse, but that doesn't mean its citizens are completely behind the times. Quite the opposite. They are focused on progress and that can only be helped by the arrival of the super-humans. Bat-Man, Accelerated Man, The Wonder Woman, The Shrinking Man and others all stand ready to defend the Modernist world and its values. If any Earth is prepared to handle unusual and trying challenges, it's Earth-19.

EARTH-38

Superheroes have existed for at least a decade on Earth-38, starting with Superman.

Years later, his cousin Supergirl would decide to come out of hiding and don the cape and tights, leading to a new heroic age.

At present, besides Superman and Supergirl, the world is also home to Martian Manhunter, Mon-El/Valor, Miss Martian, and Guardian (James Olsen).

ABBERATIONS

There are plenty of Earths created by time travel on The Flash and DC's Legends of Tomorrow -- they're generally pretty short-lived, but they exist in the multiverse at least for a time -- as a result of abberations.

That's what the series calls chronal anomalies that create alternate timelines which vie for dominance with Earth-1.

The most obvious of these is the Flashpoint reality, which Barry created by traveling back in time to save his mother's life and then lived in for a year before he tried to fix the timeline and created the current, post-Flashpoint version of Earth-1.