The Crisis on Infinite Earths was a 12-part maxi-series published by DC Comics between 1985 and 1986, written by Marv Wolfman and illustrated by George Perez. It was a story that spanned about 6 ‘universes’, featuring hundreds of characters, killed dozens of heroes and villains and changed all of time, space, the details of personal events and broader history while realigning the interconnected relationships and continuity of many DC characters. All DC Comics published thereafter are considered “Post-Crisis”; stories preceding it are “Pre-Crisis.”
It was a brilliant, unprecedented and flawed effort. Most of the problem came because of a (frankly dumb and against writer Marv Wolfman’s wishes) editorial directive at the time that required all the heroes to remember the Crisis immediately afterwards. The other problem was that many character’s “new” streamlined histories were pretty convoluted to begin with and they continued to exist without adequately explaining why, or how they specifically impacted another character’s existence (Wonder Girl, aka Donna Troy, for example). All this information kept cropping up in new stries, and in a few years DC would have to do another mini-series, Zero Hour, to clean up Crisis.
Superman is a pretty good example of a definitive “Pre-Crisis” and “Post-Crisis” character. Prior to the Crisis, he was an near infinitely powerful being who could travel through time and cross-dimensional barriers at will. He began his career as a Superboy, then became Superman, acquiring along the way, an Arctic Fortress of Solitude, super-pets, a legion of android look-alikes, an occassionally super-powered pal named Jimmy Olsen, a miniaturized city, membership in a future superhero legion and other improbable character developments. The Crisis (initially) changed all that.
The third incarnation of Superman, the current-Post Crisis version, is nowhere near as powerful. Also, gone were (again – initially, anyway) are the many interesting character developments, artifacts, whimsical adventures and powers of the Pre-Crisis version.
For more information, I refer you to:
http://www.io.com/~woodward/chroma/crisis.html#what