The following is only intended to cover DC’s history changes over the years in the broadest strokes. If you try to apply them more specifically to characters such as Superman, who had Earth-1 elements showing up in his stories at a time when his early stories were said to be taking place on Earth-2, this essay would never see then end. I’ll also gloss over the stuff that’s been repeated forever such as how superheroes came to be at DC. Almost every mainstream comic book history, even many focussed on Marvel goes into that.
Originally there wasn’t much by way of continuity. And even when there eventually was, it was pretty straight forward. If, say, the villainous Solomon Grundy was sent to the moon in a Green Lantern story and was then used in a Justice Society story (the Justice society being a team Green Lantern belonged to in the 1940s) there would be a quick explanation as to his return and then the earlier story would never be discussed again.
There was a period of time in which superheroes actually become unpopular (late 1950s to early 1960s). They didn’t disappear entirely at DC (Aquaman and Green Arrow appeared in anthology titles, Batman and Wonder Woman continued on in their own title and others and Superman actually added new spin-off titles) but for the most part superheroes ceased to be DC’s bread and butter.
Then in the 1950s, a new version of the Flash first appeared. It was there that it was established that the older Flash stories took place in comic books that the new Flash like to read. Newer versions of other heroes started to appear such as Green Lantern and Atom, and one can assume that the comic book solution applied to them as well. Never mind that this solution didn’t entirely work for characters like Superman (Lois: “Clark, I’ve been reading this comic book and there’s someone exactly like me in it. And in that comic you’re Superman…”)
The next big continuity tweak also occurred in a Flash story. The Flash learned that his “comic book heroes” existed on another Earth, with the comic book writers subconsciously tapping into events happening on this other Earth. The Earth of the current stories became Earth-1 and the older stories ironically got called Earth-2; many of course have said it should have been the other way around. As with the previous continuity explanation you had to assume that the people writing the Superman, Batman, etc. comics were writing different versions than the ones we saw, and when they tapped into Earth-2, they could somehow learn the original Flash’s secret identity but not Batman’s.
Many other Earths appeared, including Earth-Prime, which was supposed to be the real world, Earth-S, which had characters DC got from Fawcett (the “S” referred to Shazam), and Earth-X, which had heroes DC purchased from Quality. Occasionally writers got confused and/or didn’t care and some heroes like Vigilante and Wildcat appeared on the wrong Earth. In the early 1980s, the woman in charge of the letter pages of the Flash, Tamsin (I forget her surname) speculated that someday writers would try to make sense of the various Earths and that the result would be even more confusing than before.
Which is pretty much exactly what happened. Most of the various Earths and their universes of the DC Universe were destroyed early on in the 12-issue series Crisis on Infinite Earths until only five were left (Earth-1, Earth-2, Earth-4 which was the home of the newly purchased Charlton heroes, Earth-S, and Earth-X). So yes, our own Earth was apparently destroyed off panel though that was never mentioned again. Eventually the remaining heroes and villains changed history so that all the universes, including the ones that were destroyed got merged together so that in the new history there was only ever one Earth (it was mentioned in a few places that Earth-C, a home of funny animal characters was deemed a separate dimension and no longer a separate Earth, though that got changed back subsequently). Presumably elements of Earth-Prime were also part of this merger, though I don’t recall Superman existing on our world after the Crisis’ completion. In any event Earth-Prime hasn’t been used since.
In the wake of Crisis the older versions of Superman, Batman etc. were erased from history and some like Superman even had brand new histories while others continued on as before. It took a few months for some of these histories to “take” but if it was just a few months it would have been easy enough to ignore a few non-canonical stories and anomalies.
Unfortunately things continued to get more confusing. Wonder Woman was relaunched as being a brand new character. Hawkman had a history change even after a series after the Crisis came and went. Superman changes were affecting the Legion of Super-Heroes stories (set in a future that had been previously inspired by Superman’s no longer existent younger self Superboy). More and more stories were about reconciling old history to the new. And writers weren’t entirely able to avoid the temptation to find ways around the one Earth rule (e.g. a pocket universe for Superboy; Armageddon 2001 brought back the concept of a malleable, not fixed future).
So Zero Hour was written in part to explain the various anomalies since Crisis and other history change resulted. Unfortunately in the process the temptation to make more changes such as to Batman’s back story proved too great, causing more stories reconciling old stories to new.
Various Elseworlds stories (stories deliberately intended to be non-canonical) had appeared since Crisis (they appeared before Crisis as well but were called Imaginary Stories then). One, a story set in the future called Kingdom Come proved too popular to ignore, with writers wanting to use elements in previous day continuity. A mini-series with a bunch of connected one-shots called the Kingdom appeared introducing the concept of Hypertime. The premise of Hypertime was that everything that had been part of DC continuity was out there in some fashion; it’s just that some Hypertimelines were more accessible than others. Some Hypertimelines folded in and out of each other, which explained different characters in different titles recalling past events differently. While a bit confusing, Hypertime was probably as elegant a solution as one was likely to get. However after a little bit of playing around with it for a year or so, DC, showing unusual restraint basically decided to just ignore this one.
Even though continuity wasn‘t in that bad of a shape at the time, DC (and Marvel) got wrapped up in producing mini-series after mini-series of big events. Fans missed some of what had been lost since Crisis, such as Earth-2 and having Wonder Woman.be a charter member of the Justice League, something that was impossible if she arrived in “Man’s World” later. Perhaps inevitably another history change resulted, this one in a series called Infinite Crisis. As the name positioning implies it largely reversed Crisis on Infinite Earths, though the Earths weren’t yet back (Wonder Woman as charter member of the Justice League was, however).
The Earths returned in the 52 issue weekly series 52. Thankfully the addition of the extra Earths was the only history change. This time they were limited to 52 Earths, a high number but not infinite. The latest bad idea was to make too many of the Elseworlds stories part of these 52, thus limiting the number of new Earths available. Then a bunch of characters from these Earths got killed in the Countdown: Arena series which would seem to defeat the point of bringing back the concept of different Earths. There was a subsequent Crisis called Final Crisis, but it didn’t really have much history changing effect like previous Crises.
This didn’t last long. DC decided once again to change history, in a series called Flashpoint (actually technically there were two history changes in Flashpoint, but the first of these was just a temporary event, not unlike Amalgam, which squished DC and Marvel characters together for a very brief period). This time the rationale was to make stories easier to understand for new readers and provide fresh jumping on points. The innovation wasn’t to change everybody’s history this time. No, as before, some characters had their history changed, some didn’t so you have old and new history intermingled again. Even more confusingly, this history change even erased previous history changing events such as Crisis on Infinite Earths. But to a large degree the new comics would continue to reference past events. So what was supposed to make the stories easier for new readers? Oh, all the new stories would have an issue #1 on the cover, even if the creative team was the same as the previous issue. So starting everything from #1 while some past events still happened and some didn’t is supposed to clear everything up. I can’t confirm or deny this because that was the point where I finally just gave up on the DC Universe.
That brings us to the present. I’m sure when there are future history changes (and with DC, it’s a “when”, not “if”) I’ll learn of them from news sites and the like.