rustydog: What's the Crisis?
JHeaton: Oh boy. It's something to make comic book geeks sorry they mentioned it.
Here's the short version. Crisis on Infinite Earths was a twelve-issue limited series published by DC Comics in 1985 and 1986 to streamline and simplify their continuity, which had gotten rather (read: unimaginably) complex over the course of fifty years. In the context of the DC Universe, the Crisis was a cataclysmic event that resulted in history being rewritten from the beginning of time. In some cases, the new history was very similar to the old; in other cases, the new history departed radically from the old. So events that are described as "pre-Crisis" are those that occurred before history was rewritten and no longer happened in the revised continuity; "post-Crisis" events are those that happened after history was rewritten.
And now, the long version. In 1938, DC Comics (then known as National Periodical Publications) published Action Comics no. 1, featuring a new costumed super-hero named Superman. Superman was an immediate smash; by the end of 1939, Supes was headlining two comic books and appearing in his own newspaper strip. The success of Superman led DC to introduced a wide variety of other costumed heroes: Batman (1939); the Flash, Hawkman, Green Lantern, and the Atom (1940); and Wonder Woman (1941). Other publishers jumped into the fray with their own super-heroes: Fawcett (Shazam!), Quality (Plastic Man), Timely (the Human Torch), and many others. The era is known as the Golden Age.
The popularity of super-hero comics eventually started to decline, and by the early 50s virtually all super-hero comics had vanished, the exceptions being those featuring the big three: Superman, Batman and Wonder Woman. Then in 1956, in Showcase no. 4, DC introduced a new super-hero with an old name: the Flash. Aside from the name and the powers, the new character had nothing in common with the old.
The new Flash was an immediate success, and the Silver Age was born. DC soon introduced updated versions of other old characters, including Green Lantern, Hawkman, and the Atom. As was the case with the Flash, these new characters had different secret identities, different origins, and different costumes, and replaced the pulpy feel of the old stories with slick science fiction. (For example, the original Atom was just a short guy with good hand-to-hand combat skills; the new, a scientist who used white dwarf star matter to reduce himself in size.) Meanwhile, Superman, Batman, and Wonder Woman were still being published
In 1960, DC revived the idea of the super-hero team. All the Silver Age super-heroes in the DC Universe got together to form the Justice League of America, just as the Golden Age heroes had been members of the Justice Society of America. The original members of the JLA included the Flash Green Lantern, the Martian Manhunter, Aquaman, Superman, Batman and Wonder Woman. Naturally, the title was very successful, but to established something that would prove troublesome down the line: that Superman, Batman, and Wonder Woman were contemporaries of the new heroes. But they had also been contemporaries of the old heroes.
Then in 1962, the Flash responded to an emergency and found himself face to face with ... the Golden Age Flash! It turns out that the Silver Age Flash and the Golden Age Flash lived on parallel Earths, each existing at its own unique vibratory frequency. The Silver Age Earth was dubbed Earth-1, the Golden Age Earth, Earth-2. The idea proved popular, and DC embraced it whole-heartedly. The JLA and the JSA started teaming up every year, and soon other parallel Earths started popping up: Earth-3, where there were no super-heroes, only super-villains; Earth-S, where the various Shazam! characters lived; Earth-X, where the Nazis won World War II and the old Quality characters lived; and so on.
One of the big problems with the parallel Earth concept turned out to be, unsurprisingly, Superman, Batman (and Robin), and Wonder Woman. These three characters had been published without interruption since their introduction with only minor changes, and had been shown to be contemporaries of both the Golden Age and the Silver Age characters. So those three characters (plus a handful of others) existed on both Earth-1 and Earth-2. The Earth-2 versions were older, and there were a few other differences, but by and large it was difficult to tell one from the other.
The other big problem was simply that the continuity was insanely difficult to grasp. DC's big rival, Marvel Comics, was easy to understand: one Earth, one version of the most popular characters, one fairly tightly maintained continuity. Conversely, over at DC you had infinite parallel Earths, multiple version of the most popular characters, and continuity that frequently went out the window.
DC's solution to all this was Crisis on Infinite Earths, a twelve issue mini-series featuring virtually every character who had ever appeared anywhere in the DC Universe. Briefly, the Anti-Monitor, a despot from an anti-matter universe, set out to destroy the positive-matter "multiverse," and very nearly succeeded in doing so. Eventually, the heroes of five Earths -- Earth-1, Earth-2, Earth-S, Earth-X, and Earth-4 -- worked together and prevented the Anti-Monitor from wiping out those last five parallel universes by, essentially, merging them into one universe with five different Earths. Changing his strategy, the Anti-Monitor traveled back in time to the point at which the parallel universes had been created to prevent them from being formed in the first place. And following a massive battle at the beginning of time, the heroes emerged more or less victorious. But the Anti-Monitor was successful in one key respect: the creation of the multiverse, and all the heroes now found themselves on one Earth in a single positive-matter universe...
...in which many things were very different. Superman, Batman, and Wonder Woman were all modern heroes; they were not alive during World War II. New characters were created to fill their places in Golden Age stories: Iron Munro for Superman, the Flying Fox for Batman, Fury for Wonder Woman, etc. Stories that relied on cross-overs between various Earths were rewritten, or declared to have never happened. And Superman's (and Wonder Woman's, and Hawkman's, and some others) continuity was rewritten from scratch.
For example: pre-Crisis, Clark Kent started his crime-fighting career as a teen: Superboy. But post-Crisis, Clark didn't start operating openly until he was an adult. There is a post-Crisis Superboy, but he is not Superman as a boy, as was the old Superboy. Pre-Crisis, the Earth-2 Batman married Catwoman, and their daughter Helena grew up to fight crime as the Huntress. Post-Crisis, nuh-uh. Again, there is a post-Crisis Huntress, but she's not related to Batman.
Basically, it's impossible for a comics fan to discuss DC Comics properties -- such as Smallville or Birds of Prey without discussing the Crisis. Hopefully, this tutorial will help non-comics fans understand what the hell I'm talking about when I do so in the future.