Earth-One/Earth-Two: The DC Comics Multiverse was created, or at least first acknowledged, in 1961 with Gardner Fox’s seminal “Flash of Two-Worlds” in The Flash #123 (September 1961). It revealed that alongside the usual shared universe which was home to the heroes of DC’s ongoing comics there was a second universe which was home to versions of those heroes as they had been published decades before. The distinctive split between the characters came about in the late-1950s when DC decided to relaunch their virtually defunct superhero line of comicbooks. The main Earth, Earth-One, was home to the new heroes (Barry Allen, Hal Jordan, etc) and their team the Justice League of America. The second Earth, Earth-Two, was home to the older heroes (Jay Garrick, Alan Scott) and their team the Justice Society of America.
The Multivese: Multiple Earths were a brilliantly elegant concept and ,mmore Earths were added here and there through the 1960s and 1970s. The continuum of parallel worlds even gained an origin with John Broome’s “The Secret Origin of the Guardians” in Green Lantern #40 (October 1965) – a Guardian of the Universe called Krona had caused what should have been a single universe to split into a infinite number of parallel positive matter universes called the Multiverse (all permutations of the original inherently good universe) and a single Anti-Matter Universe (a place of evil). Two beings, the Monitors, were also created in that instant. The Monitor watched and guarded the Multiverse while the Anti-Monitor watched and dominated the Anti-Matter Universe.
The Crisis: The hidden war between the Monitors reached a climax in the mid-1980s with Marv Wolfman’s Crisis On Infinite Earths. In the “Crisis” the Anti-Monitor killed the Monitor and succeeded in destroying the Multiverse. However, the heroes from five worlds managed to snatch victory by partially undoing Krona’s crime. They saved reality, but at the cost of collapsing the infinite Multiverse back down into a single universe. In this new universe with its rewritten history the Justice League and Justice Society co-existed as different generations of heroes on the same Earth. Duplicates of heroes like the Flash who didn’t share a secret identity were preserved, but duplicates of the same person (e.g. Bruce Wayne, Diana Prince, Clark Kent) were merged into a single new incarnation.
Zero-Hour: There were a number of continuity problems with the merged reality which eventually led to a house cleaning series called Zero Hour (1995) written by Dan Jurgens. It was meant to give the readers a nice consistent single universe. Fans started using the term Earth-Zero for the post-Crisis, post-Zero Hour Earth based on the name of the name of Jurgens’s series. Debate has raged over the necessity of erasing the Multiverse – some thought it a barrier to new readers, others enjoyed the story telling possibilities it created. Nevertheless, it is hard to keep a good idea down for long and in the 1990s DC started producing a series of annuals, mini-series, and specials under the Elseworlds imprint. These were blatently parallel universe versions of the DC Heroes – just without the acknowledgement of the Multiverse framework.
Hypertime: British writer Grant Morrison seems to have become the greatest champion of the Multiverse. Shortly after the original Crisis he mined its implications for his acclaimed meta-textual run on Animal Man. He was also credited with the idea of Hypertime, a revised and slightly more complex version of a Multiverse that briefly appeared in some post-Zero Hour DC Comics. Karl Kesel’s “Hypertension” story arc in Superboy #60-66 (1999) and several Mark Waid stories made use of Hypertime, but it doesn’t seem to have been pursued too enthusatically by the company. Notiably Morrison, who is credited with the concept, never actually wrote a story explicitly using Hypertime.
New Earth/52: The classical Multiverse was revisited in Geoff Johns Infinite Crisis (2005) when Superboy-Prime and Alexander Luthor, a pair of refugees from the original 1960s/70s Multiverse, tried to destory the single Earth by splitting it back into its constiuent parts. They were defeated, but there were repocussions. There were subtle differences between the old and new universes, so much so that DC started refering to their post-Infinite Crisis setting as “New Earth.” Spinning out of Infinite Crisis was a year-long, weekly comicbook series called 52 written by Grant Morrison, Geoff Johns, Mark Waid and Greg Rucka. The buried mystery behind 52 was the significance of the number itself. The answer was revealed in the final issue when it was shown that New Earth wasn’t actually the only Earth to survive the second Crisis. There were 51 other Earths (52 in total) which had come into existance at the same time. They had originally been identical to the New Earth, but the flight of Mister Mind altered them at a primal level causing differences in tone and identity to develop between them.