The series (often referred to as simply "The Crisis") was one of three major comic book stories published by DC in the same year that had a profound effect on the comic book industry as a whole. While Alan Moore's Watchmen and Frank Miller's The Dark Knight Returns were widely praised and credited with giving mainstream acceptance to comic books as a form of serious literature (rather than "children's entertainment"), '\'Crisis'' was a success from a marketing point of view. It was rooted firmly in the cliché-ridden stereotype of "superheroes battle to save the world;" nevertheless, it was an exceptionally well-written and -drawn example of the costumed superhero genre, and an entertaining story in its own right. It successfully returned DC to its position as a major publisher of superhero comic books (the field had largely been dominated by Marvel Comics in the late 1970s and early 1980s).
Crisis also created the formula of the "cross-over" comic book series. Since the series was published, both DC and Marvel have had annual "summer crossover" series designed to tie many of their comic book titles together under a single storyline (and thus sell more comic books). These crossover series, sporting such titles as Millennium, Invasion!, The Final Night, and Zero Hour have boosted comic book sales, though from a storytelling point of view they have generally been inferior to Crisis. Marvel initiated its own series of annual crossovers at the same time, beginning with Secret Wars and continuing with its own "epic storylines" including The Fall of the Mutants, Acts of Vengeance, and The Infinity War.
Warning: Spoilers follow
The story introduced readers to two near-omnipotent beings, the good "Monitor" and the evil "Anti-Monitor" (both of whom died, or were destroyed, before the series finished). The Monitor had made cameo appearances in various DC comic book series for two years preceding the publication of the series, and at first he seemed to be a new supervillain; but with the onset of the "Crisis" he was revealed to be working on a desperate plan to save the universe -- not just one universe, but the entire multiverse -- from destruction at the hands of the Anti-Monitor. The Crisis series highlighted the efforts of DC Comics' superheroes to stop the Anti-Monitor's plan to destroy and conquer the various dimensional versions of the universe. Under the initial guidance of the Monitor, a select group of heroes were assigned to protect massive "tuning forks" designed to hold off a wave of antimatter (unleashed by the Anti-Monitor), that had already annihilated untold numbers of alternate Earths. Eventually, the conflict grew as nearly every DC hero got involved in the battle . The Monitor himself was murdered by his servant, Harbinger, whose mind was being controlled by the Anti-Monitor.
The Monitor's death allowed a release of energy which allowed the last five parallel Earths (the home of the DC Universe) to survive long enough for the heroes to lead an assault on the Anti-Monitor. The attack was successful to make the villain retreat, but at the cost of the death of Supergirl. An apparent lull in the war provided some breathing room for the heroes, but at this time the various DC supervillians joined forces under Brainiac and Lex Luthor to attempt the conquest of Earth, while the second Flash died stopping the Anti-Monitor's back-up scheme of destruction. The Spectre called a halt to the battles on the Earths with a warning that the Anti-Monitor was planning to travel to the beginning of time to prevent the Multiverse's creation. The DC Heroes and villains joined forces in response with the heroes traveling to stop the Anti-Monitor, and the villians traveling to the planet Oa in antiquity to prevent the renegade scientist, Krona, from performing a historic experiment that would allowed Anti-Monitor to succeed in his efforts.
In the resulting battle the villains failed at their objective, and Krona proceeded with his experiment, while the heroes supported the Spectre, who grappled with the Anti-Monitor which created a energy overload that literally shattered space and time. With that, a single DC Comics universe was created and all the superheroes found themselves in one reality where the various elements of the five Earths were fused into one. The Anti-Monitor attacked one last time, but fell to a carefully planned counter-attack devised by the DC heroes with some quiet help from the New Gods' adversary, Darkseid.
Readers unfamiliar with the complicated continuity of the DC universe may find the story of Crisis on Infinite Earths confusing, as it was written especially for readers who were intimately familiar with the many hundreds of characters created in the pages of DC comics over the space of fifty years.
Unlike the later crossover series, Crisis was used by DC as an excuse to wipe much of its slate clean and make major "changes" to many of their major revenue-generating comic book series. While some titles such as the Batman series were largely unaffected by Crisis (though The Dark Knight Returns had a considerable effect of its own on the Caped Crusader), others titles such as Green Lantern were profoundly affected. Several major DC titles were literally started over from scratch in the months following Crisis, including Wonder Woman, The Flash, Legion of Super-Heroes, and DC's flagship title Superman. While some of these "re-vamps" of classic superheroes were less successful than others, their new beginnings can generally be attributed to the success of Crisis on Infinite Earths and helped revitalize DC Comics as a major player in the comic book medium. None of the crossover series since Crisis have had such an earth-shattering effect on the histories of so many well-known comic book characters in such a short period of time.
Zero Hour (1994) is more-or-less a sequel to Crisis. It came about because the new continuity still suffered from some problems - in particular DC had continued to feature the 'old' versions of characters until new versions were launched, but some new version did not debut for many years and when they arrived continuity holes surrounding their previous appearances arose. The character of Hawkman was one of the worst, as a new version did not appear until 1993, raising the question of who had been running around since 1986.