Before the Silver Age of comics, comic books tended to have a loose interpretation of continuity. If previously published comics fit into a story that a creative team was currently telling, that was awesome. If it didn't, someone sat around trying to figure out why and then they published something in a comics fanzine somewhere.
But, as of 1961, there were two Flashes, and they lived on Earth-1 and Earth-2. Weirdly the heroes who were around first were relegated to Earth-2. But for the first time there was an in story rationale for the then current Flash, Barry Allen, and the original, Golden Age Flash, Jay Garrick, to more or less coexist. And if both of these Flashes lived in the same universe that meant that the two characters could team up together. Fans of the Jay Garrick Flash of the 40s could read new stories about the character they loved, and hopefully find out what he had been doing with himself in the interim.
The DC Comics weren't using the term "multiverse" just yet, that would come later after the Justice League of Earth-1 and the Justice Society of Earth-2 had been teaming up for a bit. These team ups lead to other Earths in the burgeoning multiverse: Earth-X where Nazis had won World War II and the characters originally published by Quality Comics lived, Earth-3 where history had developed in "reverse," and everyone who was good on other Earths was evil on this one, Earth-S which was home to the Captain Marvel and other characters originally published by Fawcett Comics lived, and more and more over time.
The multiverse at DC Comics grew with leaps and bounds, because writers and artists were eager to add their own concepts and characters to the multiverse. This also lead to a lot of confusion for some. When did the stories in the Superman comic stop being set on Earth-2, and when did they shift to Earth-1? Why is Black Canary, a character who started out in the 40s, still young and vital in the 1980s? What Earth did Red Tornado really come from?
The powers that be at DC Comics decided it was time to wipe the slate clean and restart the stories of the DC Universe. Worlds would live. Worlds would die. Nothing would ever be the same! Well, until the next Crisis at least. DC Comics announced they would do a "maxi-series" to celebrate the company's 50th anniversary that would clean up and reset the company's continuity. There would be one Earth, with one history. And all of the heroes would live there. That ended up being a lot of super-heroes.
The problem that sprung from this was that creators still wanted their favorite stories from the past to have existed, regardless of their being 20, 30 or 40 years old, and so they made reference to them in the stories. Things deteriorated quickly, and within a couple of years it was decided that another crisis was needed to clear up all of the previous clearing up. This crisis was a crisis of faith and confidence, and it was called Legends. With Legends the company reintroduced Wonder Woman, post Crisis on Infinite Earths, making her younger and once again new to Man's World. The character formerly known as Captain Marvel, and now known as Shazam or The Captain, was better integrated into the new Earth. Blue Beetle, a character previously published by Charlton Comics, but with a pedigree that stretched back to the Gold Age of Comics, was also brought to a new prominence within the universe published by DC Comics. A new Justice League would come out of this crisis, and it would feature the aforementioned Blue Beetle and Captain Marvel, along with Mister Miracle, a character created by Jack Kirby in the 1970s, and others.
Jack Kirby leaving Marvel Comics in the 70s to go to their rivals DC Comics was a huge deal. Kirby created a series of books dealing with gods and monsters engaged in a cosmic battle that coalesced under the umbrella of the "Fourth World." These Fourth World stories would more or less stand on their own, in their own little corner of the DC Universe despite appearances by characters like Superman or Deadman in various of the books. Post Crisis on Infinite Earths and post Legends, the creations of Jack Kirby, along with characters from other companies purchased by DC Comics over the years, would be brought together in this new Earth.
The problem then became that the world was getting pretty full of super-heroes. This lead to other crises and revisions to the DC Universe over the years. The number of Earths in the DC Comics multiverse would expand and contract with each event that the company launched. Worlds would live. Worlds would die. Nothing would ever be the same.
