Over almost eighty years of publishing history, DC Comics has gotten a reputation for mucking around rather a lot with the fictional history of its shared superhero universe. Currently it's in the early stages of "Rebirth," which (in a nutshell) apparently aims to restore ten years' worth of in-universe time to the five-year backstory of the New 52.
If the previous sentence made your eyes glaze over and your brain go numb, this post wants to help. What follows is a concise-ish catalog of how the comics company has sought to define (or redefine) its collective cosmos; and it goes back quite a ways.
The company we think of as DC Comics was formed out of three distinct entities: National Allied Publications, Detective Comics Inc. and All-American Publications. National began publishing "New Fun Comics" (later retitled "More Fun Comics") in 1935, and "New Comics" (later "Adventure Comics") in 1936. Detective Comics' eponymous series came along later in 1936 (#1 cover-dated 1937), and in 1938 it launched the industry-changing "Action Comics."
In 1946 Detective Comics Inc., National Allied Publications and All-American Publications became one corporate entity, National Periodical Publications. That company didn't officially change its name to "DC Comics Inc." until 1977, although it had been known as such for decades. I mention the corporate moves because collectively, they make the first real "DC Universe" moment even more remarkable.
DC 1.0: THE JUSTICE SOCIETY
Debuting in All-American's "All-Star Comics" #3 (1941), the Justice Society of America brought together characters from all three early entities, including National's Doctor Fate, Hourman and Sandman, All-American's Flash, Atom, Green Lantern and Wonder Woman, and (occasionally) Detective's Batman and Superman. Speaking practically, though, the fact that they came from different (albeit closely-connected) publishers is more of a historical footnote. What matters is that they were a team at all. The notion that all these characters appeared in their own titles and then met up to fight even bigger menaces is a given today, but back then it was groundbreaking.
DC 1.1: SUPERBOY
Another significant bit of history came in 1945, with the debut of Superboy in "More Fun Comics" #101. As the tagline reminded readers, these were "the adventures of Superman when he was a boy" -- but that statement arguably conflicted with the Man of Steel's origin as documented in 1938's "Action" #1, and (in expanded form) in 1939's "Superman" #1. Now, it's a fair point as to whether 1940s readers really cared as much about consistency; but eventually they would care. Boy, would they. Reconciling these sorts of differences would eventually mean creating both a whole new cosmology, and new terms like "retroactive continuity" to describe it.
DC 2.0: THE SILVER AGE
In hindsight, the start of DC's Silver Age shares a lot with a modern line-wide reboot. It reintroduced Golden Age characters like the Flash, Green Lantern, Atom and Hawkman, but changed them significantly and -- for a while, at least -- all but ignored the original stories. It also introduced new characters (Martian Manhunter, Adam Strange, the Doom Patrol); modernized Superman, Batman and Wonder Woman; and updated the JSA into the Justice League of America.
Of course, the Silver Age's two foundational events both involve The Flash. September-October 1956's "Showcase" #4 introduced the second Scarlet Speedster, Barry Allen; and February 1961's "Flash" #123 revealed that original Flash Jay Garrick had been living on the parallel world of Earth-Two. By that time it had been ten years since the last Justice Society story (February-March 1951's "All-Star" #57) and twelve years since Jay's "Flash Comics" had been cancelled.
DC 2.1: THE MULTIVERSE
Thus, right from the start Earth-Two laid out a couple of ground rules that would distinguish itself from Barry's Earth-One. It presumed that all of Jay's Golden Age adventures had "actually" happened (just on Earth-Two); and also that they happened in real time (i.e. from Jay's introduction in 1940's "Flash Comics" #1 to his 1949 retirement).
The rules of Earth-Two also ended up defining the main Earth-One. The five-and-a-half years between "All-Star" #57 and "Showcase" #4 included some stories that would become part of Earth-One history. Among them were the debut of Captain Comet in June 1951's "Strange Adventures" #9; the start of the Superman/Batman team-ups in May-June 1952's "Superman" #76; and the Martian Manhunter's first appearance in November 1955's "Detective" #225. The Superboy stories were also retroactively assigned to Earth-One -- or at least they were deemed not to have happened to the Superman of Earth-Two, who didn't appear in a Silver Age story until 1969's "JLA" #75.
Otherwise, the early 1950s were pretty quiet for DC's superheroes. Superman, Batman and Robin, and Wonder Woman survived the end of the Golden Age, but except for the aforementioned team-ups (monthly in "World's Finest Comics") they didn't interact with each other. Those team-ups deserve another mention, because "Superman" #76 purported to tell the first meeting of Superman and Batman, despite the two being members of the JSA (and having shared adventures on the 1940s "Adventures of Superman" radio show). As with the Superboy stories, this wasn't just a contradiction of Golden Age history, but a piece of Earth-One history; and that conceit helped to establish the boundaries between Earths.
Naturally, Earth-One history only got more expansive as the Silver Age went on. By the 1970s its heroes had gotten an elastic timeline which kept them perpetually young (or at least young-ish). This was in contrast to Earth-Two, whose Golden Agers were pinned inflexibly to the 1940s, and aged accordingly. Such distinctions became necessary elements of the Multiverse which DC's creative teams were gradually building.
DC 3.0: YOU-KNOW-WHAT
By the mid-1980s, conventional wisdom claimed that DC's now-infinite Multiverse had become too unwieldy, so 1985's "Crisis On Infinite Earths" sought to streamline it. Smooshing together the last universes standing created a singular timeline which accommodated most of their characters. The ostensibly-streamlined DC-Earth had a Golden Age which started in the late 1930s (albeit without a Superman, Batman or Wonder Woman), a Silver Age which started "seven years ago," three distinct generations of super-folk, and quite a few super-people who'd been published originally by companies other than DC.
DC 3.1: ROLLING REBOOTS OF THE LATE '80S
Although "Crisis" ended in November 1985, DC wasn't done rebooting and/or relaunching. Starting with June 1986's "Man of Steel" #1 and lasting well into 1987, DC rolled out revised versions of Superman, Wonder Woman, Batman, Blue Beetle, Captain Atom and Captain Marvel. During the same period it relaunched "Justice League," "Suicide Squad" and "Flash" (now starring Wally West). By the time Tim Truman tackled Hawkman's beginnings in "Hawkworld," it was 1989 and the new DC Universe was almost three-and-a-half years old.
Furthermore, DC creative teams had found new ways to tell parallel-world stories. The Superman reboot did away with a "Superboy" career, but the Legion of Super-Heroes' history hadn't changed; so a 1987 crossover explained the discrepancy with a "pocket universe." It wasn't a parallel Earth, just a sliver of time cultivated and shaped to look like the pre-Crisis status quo. (A time-twisting 1990 "Legion" issue then removed Superboy and Supergirl from Legion history and replaced them with Lar Gand and Laurel Gand.) Similarly, in 1987 the Justice League encountered a group of Avengers analogues from the "other-dimensional" world of Angor -- which, of course, happened to look and act just like a parallel Earth.
DC 3.2: ZERO HOUR
Hawkman wasn't the only problem. Besides Superman and the Legion, changes to Wonder Woman affected the histories of the Justice League and Teen Titans; and a plot point in "Batman: Year One" required a change in Batgirl's background. In the summer of 1994, writer/artist Dan Jurgens (with inker Jerry Ordway) produced the five-issue weekly "Zero Hour: Crisis In Time!" It played with alternate timelines instead of parallel worlds, but with a familiar ending: everything was the same, except for the little differences. "Zero Hour" tried its best to fix Hawkman, retired the Justice Society, and rebooted the Legion of Super-Heroes with a pair of zero issues and a young-again cast. That Legion reboot was probably the most significant product of post-"Zero Hour" continuity, with the new "Starman" series a close second; and "Starman" had next to nothing to do with "Zero Hour" beyond a cameo in the miniseries. For the rest of the superhero books "Zero Hour" was basically a collective deep breath and a chance to regroup.