Pre-Crisis / The Multiverse
Inside the DC Comics mythology, the sci-fi concept of alternate realities existing as parallel earths has been one of the main characteristics of their superhero universe since the silver age. With an infinite number of similar looking universes than resemble the main universe, the idea of Multiverse is a foundation concept not only in DC Comics but for the whole superhero genre.
Before the Crisis on Infinite Earths, there were many (hence “infinite”) versions of the Earth scattered throughout the Multiverse. The DC Multiverse officially first appeared in The Flash #123 when Barry Allen aka The Flash found himself in another universe that was separated from his own by being on a different vibration frequency.
This world came to be known as Earth-Two and was populated by the heroes of the (Golden Age) of comics the 40’s and 50’s. After this, more worlds were found, and then even more, until finally DC decided it was time to put an end to the expansion of the Multiverse. As was mentioned before, the other worlds were destroyed during the Crisis, but here is a list of the known Earth’s before they were lost.
Post-Crisis / DC Universe
During the period than the DC was reduced to one clustered universe, embers of the previous reality popped once and again in different titles by different writers, always searching a way to bring back the old multiverse in some fashion, but never calling them as parallel earths. Such is the case of the Second Crisis from Grant Morrison in Animal Man, Mark Waid in Kingdom, Jeph Loeb in Superman/Batman. During this age, the alternative stories were dubbed (Elsewhere), such as Batman: Gotham by Gaslight and Superman: Red Son, with some of them becoming official alternate universes.
Infinite Crisis / The Multiverse
During the epic Infinite crisis, Alexander Luthor, one of the few survivals of the original multiverse briefly recreated some of the previous existing worlds, trying to find the “perfect earth”. Although his schemes were thwarted and this multiverse was brief in existence (two issues), it would be the first step for the return of the parallel earths.
Post-52 Weeks / The Multiverse
Born after 52 (the weekly) and modified by Mr Mind, this multiverse resemble previous incarnations. Various Elseworlds were part of this multiverse. It would be explored more in Countdown main series and parallel spin off as Lord Havok, Crime Society, Wildstorm, Red Rain, GaslIght, Red Son, Arena.
Post-Flashpoint / The Multiverse
The New 52 Multiverse is being explored in Multiversity. Although works similar to previous incarnations (by the earths occupying the same space but vibrating differently), there is a map showing the nature of the multiverse with worlds occupying specific zones divided by axis Order-Chaos & Pinnacle&Pit , which describes the nature of each earth.