Crisis on Infinite Earths occured in the 80s. You can pick up the trade paperback with a wonderful wrap-around cover that has every DC character on it.
Pre-Crisis: There were an infinite number of Earths on which stories could occur. Earth-1 had the Silver Age heroes, Barry Allen (Flash), Hal Jordan (Green Lantern), Superman in his prime at his most powerful, and the Justice League of America. Earth-2 had the Golden Age heroes, Jay Garrick (Flash), Alan Scott (Green Lantern), Superman with some grey in his hair and more based on the original Superman concept, Batman had died, Dick Grayson (Robin) has grown up, and Bruce Wayne�s daughter is a heroine (Huntress), and the Justice Society of America. Earth-S (I think) housed the Marvels, Captain Marvel, Mary Marvel of Shazam fame, not the comic book publisher. There was an Earth that held the Charlton company�s heroes as well, since DC bought them out.
Crisis got rid of the multiple realities, reducing the DC Universe to a single Earth, and putting all of the heroes into their historical places within the one world. Superman, Batman, and Wonder Woman were the major heroes whose birth was changed. Instead of being active in the 40s as the first wave of heroes, these three were moved back to the second wave of heroes. The Justice Society was active during World War II, then disappeared during McCarthy�s hunt for commies. Superman�s appearance in Metropolis kicks off the new age of heroes, quickly leading to the new Justice League.
Post-Crisis: The biggest change from the Crisis in a single character, I think, is the revamp of Superman. Okay, actually the deaths of both Barry Allen (Flash) and Supergirl would be the biggest changes, but I'm talking about changing the past of the hero. Done by John Byrne, Superman�s whole history is changed. First off, he is the SOLE survivor from Krypton. No super-pets. No Supergirl. No bottled city. Only Superman (back to his origins, in this way). Also, he was greatly depowered. Story telling was difficult with a godlike character like Superman (especially since the writers, not being super-intelligent, had a hard time showing Superman being super-intelligent). Clark Kent now grows up with no powers as a popular star athlete in Smallville. He discovers his powers late in high school, travels the world, then appears in Metropolis to meet a Lex Luthor much older than he is. It is due to these vast differences in the Superman mythos that you will see Pre-Crisis and Post-Crisis referred to alot by comic book readers discussing Superman.
Crisis itself is the result of a powerful villain, the Anti-Monitor. The Anti-Monitor decides to wipe out all of the universes. He's some kind of antimatter character who will be all powerful when the regular universes cease to exist. He is opposed by the Monitor, who cryptically gathers heroes and villains from multiple Earths together to stop the destruction. One by one, the Earths are destroyed, but the Monitor saves Earth-1 and Earth-2 (I think) despite dying himself.
In the end, if I remember correctly, Earth-1 and Earth-2 are also destroyed, but the universe is restarted through a big bang, this time without splitting into multiple universes. This results in the singular timeline evolving into what exists today. (I may be confusing the ending of this story with a few others, though)
DC has made a regular practice of restarting time through a big bang triggered by the heroes. They did it again during Zero Hour, and at the end of Our Worlds At War they messed with the big bang it seems. Basically, whenever the villain is too successful, it's time to restart time. In fact, the multiverse of pre-Crisis was created in a Green Lantern story (recounted by Larry Niven, I think, in "Green Lantern: Ganthet's Tale") where an scientist from Oa monitors the big bang and tries to harness the energy, accidentally splitting it into an infinite number of universes existing on separate vibrational frequencies.
