In the beginning, the Multiverse was Infinite.
A few of these worlds were explored. Earth One was Silver Age and onward. Earth Two was Golden Age, where the Justice Society of America protected people. Earth 3 is the evil mirror universe, and the others there are properties that DC bought.
Then Crisis on Infinite Earths happened, and most of the Multiverse was obliterated by the Anti-Monitor.
Only a handful of Earths survived, but they weren't stable after an attack from the Anti-Monitor, so they had to be merged.
Histories, characters, landscapes, all happening on one New Earth.
And this worked until the mid 90s, when and insane Hal Jordan calling himself Parallax hatched a scheme to give himself near limitless power and alter history.
And the hell of it is, he succeeded. His home city was restored, and many histories were slightly altered.
Next was the Infinite Crisis.
A handful of survivors of COIE, displeased with the current landscape, built a great machine to recreate the multiverse, in search of the Perfect World.
They didn't find it, but the Multiverse was restored, though at the moment, it was just 52 of the same thing.
Now comes 52, a weekly story directly following Infinite Crisis and the year when Superman, Batman, and Wonder Woman were inactive.
At the end, the mastermind behind most of the events in the book got into the Bleed, the space between Multiverses, where he could feed on their histories form the outside.
This made most of the worlds different.
Countdown happened, and explored a few of them.
Final Crisis folded Milestone into the main universe somehow.