Yesterday I rambled on about issue #6 of Infinite Crisis. Regular reader Jey left me a commment on that post, asking in part "what per se is the infinite crisis?"
Seeing as I'm probably going to review (read "rant about") some other Infinite Crisis titles in the future, I'll quickly answer that with a full post.
Basically, there are two main "key figure" publishers in the field of superhero comics. One is Marvel Comics, which features a stable of characters including Spider-Man, the X-Men, the Avengers and the Fantastic Four. The other is DC Comics (the DC originally stood for Detective Comics) who plays home to Batman, Superman, Green Lantern, and the Justice League of America, among many others.
There has been a longstanding tradition in superhero comics, dating back to the appearance of the Justice Society of America in All Star Comics back in the 40s, for superheros owned by the same company to exist in the same world, or "universe" as each other. The economic benefits of this are that the publisher can prop up new or faltering comics with an appearance by a more popular character (like Superman or Batman), and also (unfortunately) that a large stable of established characters can be used as supporting characters and villains without having to be fully introduced, thus saving on the page count for already quite short issues. Thus, the comics "crossover".
However, despite this intention of a single coherent universe, DC found in the late 70s and early 80s that it had achieved anything but. In addition to the regular "DC Universe", it had acquired a number of properties from a variety of other publishers who had gone out of business over the preceding decades, including the once popular Shazam and Blue Beetle. What was more, even within its main DC Universe it had created the precedent of a variety of "alternate Earths". Begun as a way of trying to bring its Golden Age (40s and 50s) era characters into regular guest appearances during the 60s and 70s, this had rapidly spun out into a seemingly never-ending parade of alternate dimension stories and "imaginary stories", to the point where a ridiculously small amount of action in some of its core titles was actually happening in the "real world".
In the early to mid 1980s, DC Comics launched a crossover event on a scale never before tried. They would do a 12 issue series, featuring virtually every character from the DC Universe past or present, plus some surprise guests, publishing over the course of a year, that would fundamentally reinvent the nature of the universe. This story was called the Crisis on Infinite Earths. In a plot that still makes very little sense more than 20 years after the event, the "multiverse" of alternate Earths was condensed into (allegedly) one single Earth, incorporating elements from all the others. The Golden Age JSA now co-existed alongside their modern namesakes; the Marvel Family and the Wizard Shazam joined the DC Universe, along with a host of other "refugee" characters; and a variety of characters were "relaunched" with important "improvements" to their backstory (most notably Batman, who benefited very strongly from his new darker image).
In addition, the Crisis was meant to straighten out a bunch of lingering continuity problems relating to some characters. It killed off or wrote out of existence the extended "Superman Family", including the Bottled City of Kandor, the League of Super-Pets, and Superman's cousin Supergirl, leaving Superman once again the last son of Krypton. The Silver Age Flash, Barry Allen, was killed, and his former sidekick Wally West took on the name and mantle. And the Golden Age version of several characters, including Superman, Wonder Woman, and Superboy vanished into the narrative gulf. The series was a massive success, breaking all sorts of sales records, selling by the truckload.
Much has passed under the bridge since then. It quickly became apparent that the Crisis had caused more continuity problems than it had fixed. In particular there were severe problems relating to the identity of Hawkman, and the relationship of Superboy to the Legion of Super-Heroes (issues too complex to discuss here). A series of mini-crisis events attempted to address this (Zero Hour, Final Night) with little success.
Supergirl was returned to comics various times with mixed results - we've seen her as an angel, as a protoplasmic being, and recently as a new version of "Superman's cousin". In addition, there's a character called Power Girl, who's variously claimed to be a hero from the past sent through time by an Atlantean wizard, or possibly also another survivor from Krypton.
Superboy, Krypto the Wonderdog, and the Bottled City of Kandor have also all returned (although thankfully still no sign of Supergirl's Super-Horse). The situation regarding Hawkman's identity has just got more and more confusing, and now apparently there's some issues with continuity of the cult favourite Doom Patrol that need attention.
So now DC have, over the last couple of years, been running this big "Infinite Crisis" thing. Almost everything in the DC Universe recently has been leading into this. It turns out that the Golden Age Superman (also known as the Earth-2 Superman) did not vanish during the original Crisis, but has instead been hanging out in an extradimensional prison along with several other Crisis survivors, including the Earth-2 Lois Lane, the Earth Prime Superboy, and the Earth-3 Lex "Alex" Luthor (who was originally a kind of hero introduced during the first Crisis).
Having been watching the DC Universe all these years, these survivors have finally decided that the wrong Earth was saved during the Crisis. Earth's heroes have been getting progressively darker and grittier of late, culminating in Wonder Woman murdering villain Maxwell Lord to prevent him from doing further harm. Superman et al want to re-instigate the Crisis and this time save the "Golden Age" world, returning us all to a time of sunshine and buttercups where good was good and bad was bad and you could tell the difference between right and wrong, dammit.
This is all complicated by revelations that some members of the Justice League of America have been mindwiping villains for years; a rogue satellite with a knowledge of the real identities of all metahumans on Earth and an army of killer robots at its command; the Spectre (the spirit of vengeance) engaging in a pogram against all magic; and an interplanetary war.
During the course of events so far, we've seen the Blue Beetle killed, we've discovered Power Girl is the original Supergirl, we've seen Earth Prime Superboy go mad and literally rip apart a bunch of Teen Titans, we've seen Jade killed and Kyle Rayner transformed into a being called Ion, we've seen Sasha Bordeaux turn into a new cyber-thing that it's hard to care about... and I think Hawkgirl may be dead. The Spectre has a new host, Deathstroke is suddenly cool, Bludhaven got nuked, the Wizard Shazam died, there's a new Blue Beetle, a new magical superteam called Shadowpact has formed, Jason Todd's returned to life, Superboy and Wonder Girl had sex, and all kinds of hell has broken loose.
The outcome is a vast money-spinner for DC with the potential for them to make some continuity quick-fixes to a bunch of titles that didn't really need it, and relaunch some deathly dull titles with an all-new all-exciting deathly dull continuity.
I hope this has been instructive; stand by for further news.