In the late 30s, 40s, and early 50s the company that would become DC Comics published a great many superhero titles, featuring characters such as Superman and the Flash (Jay Garrick). Many of them were members of the Justice Society of America. This era is now called the Golden Age.
In the late 40s, superhero popularity declined, and through the mid-50s only Superman, Batman, Wonder Woman, Aquaman, Green Arrow and a precious few other heroes remained in publication.
In 1956, a new Flash (Barry Allen) was introduced, inaugurating the Silver Age of comics. Soon There were new version of Green Lantern, Hawkman, the Atom, and others.
Barry Allen was in part inspired to become "Flash II" by the comics he had read as a child; comics about Jay Garrick, "Flash I". Flash II later joined the Justice League of America, alongside Superman, who was, of course, still around.
The problem: Superman had fought beside Flash I; Superman was now fighting beside Flash II. But, to Flash II, Flash I was a fictional character. The "fictional" aspect was addressed a few years later, in the pivotal story "Flash Of Two Worlds" (Flash #123). In it, Flash II accidentally "tore a gap in the vibratory shields separating [two] worlds", and traveled to an alternate Earth, one where the retired Jay Garrick lived. When asked how Barry could possibly have read about Jay's adventures, Barry replied, "A writer named Gardner Fox wrote aboutyour adventures -- which he claimed came to him in dreams! Obviously when Fox was asleep, his mind was "tuned in" on your vibratory Earth!"

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In Justice League of America #21-22 was "Crisis On Earth-One!" and "Crisis On Earth-Two!", in which the Justice Society and Justice League teamed up to foil a cross-Earth group of villains who had found a way to bridge the gap through music and magic. The issue of the "identical duplicates" was ignored - Superman did not appear to remember having been a member of the Justice Society, for instance. The Silver Age Earth of Barry Allen was dubbed "Earth-1", and the world of Jay Garrick, "Earth-2".
At the very end of the adventure, as the heroes are closing in on the villains, their thoughts turn to escape, and the Fiddler cries out, "There is an Earth-One and an Earth-Two! Somewhere there must be an Earth-Three! If we can find the doorway into it -- before the justice champions find us -- we can escape them forever!" They did not find it in time, but the seed of a multiverse was planted.

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Over the following decades on the order of two dozen Earths, out of a presumed infinity, were discovered or described. Furthermore, the "identical duplicate" problem was explained - there were two Supermen, one on Earth-2, member of the Justice Society, one on Earth-1, member of the Justice League. Several other characters also had identical duplicates, while some merely had "similar" equivalents, like the two Flashes.
Eventually other Earths were created, such as "Earth-3" [where the heroes' and villains' roles were reverse, so "Ultraman" was the villain and Lex Luthor was the hero], and new Earth's were added to the DC continuum when DC comics bought out other comic book companies' characters [like Earth-S for the (Shazam) Marvel Family].
On Earth-2, Superman married Lois Lane, Batman married Catwoman (and their daughter became the Huntress. Their Batman also died, after a stint as Police Commissioner of Gotham.

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Starting in New Teen Titans Annual #2, a fellow known as the Monitor is shown in his hidden satellite. He acts as an intermediary for master criminals who need super-thugs. He later goes on to appear in just about every DC Comic (including Sgt. Rock). He had a beautiful assistant, Lyla.

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By 1985, the abundance of Earths had become unwieldy. They also wanted to discard the "baggage" of continuity that some of their characters had acquired. So, Crisis on Infinite Earths.

Marv Wolfman credits the start of the project as a letter column in GREEN LANTERN vol2 #143.

Peter Sanderson was hired to read ALL of DC's fifty years worth of material and make notes. Bob Greenberger was brought into help him. Lists of characters were drawn up. Notes on stories and dimensions were written. Much of the material would help form the companion WHO'S WHO title. Then more lists were drawn up, which characters would die, who would be altered and who would be created. The history became DC Universe: Crisis on Infinite Earths and then just Crisis on Infinite Earths.
The title is in reference to the above-mentioned issues of Justice League. Crisis served to "simplify" their multiverse by bringing it down into a single universe, with all the heroes. They also decided to use this as an excuse to kill off, not only "redundant" characters (like the Superman of Earth-2), but some of their "stars" like Supergirl, and the Flash. The history issues were later done in the History of the DC Universe.
Crisis involved more characters than most crossovers. It spanned the Dawn of Time to the 30th Century, and included Old West characters, war characters, sf characters, historical characters, anthropological characters, and a half-dozen other genres that are absent from modern comics.
Dozens of heroes were grief-stricken when their realities, their loved ones, were lost in the merger. An infinite number of people died. The current DC Universe is built on a foundation of corpses.

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