Event

Crisis on Infinite Earths
What Happened

At the dawn of time and space a cataclysmic explosion created a cosmic current sparking the transformation of the one universe into a Multiverse, an infinite number of parallel universes existing in the same place in time, but separated by barriers of vibrations at different harmonic frequencies.  Originating from the planet Oa the cataclysm created the Qward universe, a dark dimension of anti-matter where evil always prevails.  Both positive-matter and anti-matter realms had their champions, near-omnipotent beings respectively known as the Monitor and the Anti-Monitor.

Dormant for nine billion years, the Anti-Monitor was awoken when the scientist who would become known as Pariah unleashed Qward's anti-matter energy into his positive-matter universe.  The breach destroyed Pariah's universe and began the systematic destruction of the Multiverse.  Pariah's breach into the anti-matter universe also awoke the Monitor, who gathered a battalion of select heroes and villains from multiple dimensions in a hopes of stemming the Anti-Monitor's assault and save the the positive-matter universes.

Aided by the Monitor's assistant, Harbinger, those selected from the five Earths battle to save the universe from the Anti-Monitor's chaotic energies.  The Monitor in turn allows his life to be sacrificed to create a neatherverse to protect the five planets: Earth-1, Earth-2, Earth-4, Earth-S and Earth-X from the Anti-Monitors schemes, but it results in the planets being partially merged.  As the war rages heroes live and die, with Supergirl II and Flash II of Earth-1, surrendering their lives to prevent the Anti-Monitor's schemes from succeeding.  Weakened by Supergirl and the Flash's separate attacks the Anti-Monitor reasons that the key to ultimate victory is at the moment of creation.  Creating a temporal vortex the Anti-Monitor travels back through the centuries waiting to ambush his foes.  Meanwhile the most powerful legion of super-villains ever assembled attempt to take control of the partially merged planets.  The battle between the heroes and villains is halted by the Spectre who has the heroes of the Multiverse challenge the Anti-Monitor at the beginning of space and time, while the villains travel into the past in an attempt to stop Krona's experiment on the planet Oa.

At the dawn of creation, the heroes of the Multiverse witness the horror of a clash between the Anti-Monitor and near omnipotent Spectre.  As the universe is ripped asunder during this titanic clash, the fabric of time is rewoven and reality appears to return to normal, until it is realized that where once existed an infinite number of parallel universes, now there is only one, combined from elements and histories of each of the remaining merged universes.  Created from this merging is Earth-Sigma, where the remaining heroes make their final triumphant stand against the Anti-Monitor, forever ending his treat to the remaining universe.  Although victorious, the effects of the Anti-Monitor's plot cannot be undone.  Mercifully a ripple in the timestream erased from memory any awareness there had ever been any universe other than one, except for a now maddened Psycho-Pirate.
Why It Happened
The Crisis on Infinite Earths was both an economic and artistic decision designed to breath new life into the characters of the DC universe as well as relieve the creative and editorial staff of a burdensome and conflicting continuity accumulated over 45 years.  The title "Crisis On Infinite Earths" came from a story arc by Garner Fox in Justice League of America Vol. 1 #21 (Aug. 1962) and Justice League of America Vol. 1 #22  (Sept. 1962), titled "Crisis on Earth-One!" and "Crisis on Earth-Two!", in which the Justice League of America on Earth-1 teamed with the Justice Society of America on Earth-2 to stop a group of villains from both Earths which had plans to conquer the planets.  This story began the concept of a multiverse.

To understand the DC Universe, readers must think of history in aspects of Pre-Crisis and Post-Crisis.  Case in point, Pre-Crisis Princess Diana, the Wonder Woman from World War II and a member of the Justice Society of America, became Hippolyta the mother of the current Wonder Woman, in effect making Princess Diana Wonder Woman II, as opposed to Wonder Woman I.  In the Post-Crisis world there never existed either a Clark Kent Superbaby or Superboy.  Kent's powers as Superman did not develop until puberty.   In the Post-Crisis world Selina Kyle (Catwoman) never married Bruce Wayne (Batman) having a daughter who would become Huntress II, nor did Dick Grayson continue his career as Robin into adulthood.  The apparent benefits of the Crisis is for the creative staff of DC Comics and die-hard fans only, because due to the nature of characters in periodic publications, only the story arcs being published by the current creators of the past few years generally have relevance to most readers.  A good example would be the James Bond 077 films.  All the characters change, except for Bond, who is obviously played by different actors, yet everyone accepts him as the same guy.
Notable Deaths, Destruction or Introduction
Died or omitted from history: Anti-Monitor | Aquagirl | Batwoman (Earth-1) | Batwoman (Earth-2) | Crime Syndicate (Earth-3): Johnny Quick II, Power Ring I, Owlman I, Superwoman I & Ultraman I | Dove I |  Flash II (Earth-1) | Green Arrow (Earth-2) | Huntress II (Earth-2) | Icicle I (Earth-2) | Justice Alliance (Earth-D): Atom, Aquaman, Batman, Flash, Green Arrow, Green Lantern I, Green Lantern II, Hawkgirl, Hawkman, Martian Manhunter, Robin, Supergirl, Superman & Wonder Woman | Luthor I (Earth-2) | Luthor III (Earth-3) | Mirror Master I | Monitor | Robin I (Earth-2) | Superboy (Earth-Prime, exile) | Supergirl II (Earth-1) | Superman (Earth-2, exile) | Wonder Woman (revamped)

Merged: Earth-1 (silver-age heroes DC) + Earth-2 (golden-age heroes DC) + Earth-4 (Charlton Comics) + Earth-S (Fawcett Comics) + Earth-X (Quality Comics) = Earth-Sigma

Destroyed: Earth-3 (evil dimension), Earth-5, Earth-6, Earth-12 (Inferior Five), Earth-A, Earth-B (dimension for unexplained continuity gaffs), Earth-D, Earth-E (era between gold and silver age), Earth-K (Kamandi timeline), Earth-Omega (home of Pariah), Earth-Prime (real world), Earth-Quality (Quality Comics)

Spared: Earth-C (funny animals), Earth-C minus (more funny animals), Fifth Dimension (Imp dimension), New Gods Dimension, Qward (silver-age anti-matter universe)

 Introduced into continuity: Charlton Heroes of Earth-4: Blue Beetle II, Captain Atom, Judomaster, Nightshade, Peacemaker I, (The) Question, Sarge Steel

Recently revived: Earth-2 Crime Syndicate of Amerika: Johnny Quick III, Power Ring II, Owlman II, Superwoman III & Ultraman II - JLA: Earth 2 (Dec. 1999)
Creators
Marv Wolfman & George Pérez
Pertinent Issues in Order

All-Star Squadron #50-60 (Oct. 1985 - Aug. 1986)

Batman #389-391 (Nov. 1985 - Jan. 1986)

Blue Devil #17-19 (Dec. 1985 - Feb. 1986)

Crisis on Infinite Earths #1-12 (April 1985 - March 1986)

DC Comics Presents 78, 86-88 & 94-95 (Feb. 1985, Oct. 1985 - Dec. 1985 & July 1985)

Detective Comics #555-558 (Oct. 1985 - Jan. 1986)

Green Lantern Vol. 1 #194-198 (Nov. 1985 - Feb. 1985)

Infinity Inc. #18-25 & An. #1 (Sept. 1985 - April 1986)

Justice League of America #244-245 & An. #3 (Nov. 1985 - Dec. 1985)

Legends of The DC Universe: Crisis On Infinite Earths #1 " Crisis On Infinite Earths #4 ˝ " (Feb. 1999)

Legion of Super-Heroes Vol. 2 #13 - 18 & An. #1 (Aug. 1985 & Jan. 1986)

New Teen Titans Vol. 2 #13-15 (Oct. 1985 - Dec. 1985)

Omega Men #26 - 33 (April 1985 & Dec. 1985)

Superman Vol. 1 #413-415 (Nov. 1985 - Jan. 1986)

Swamp Thing & Saga of Swamp Thing #39 & 46 (Aug. 1985 & March 1986)

Wonder Woman Vol. 1 #327-329 (Dec. 1985 - Feb. 1986)