Crisis On Infinite Earths

"I'm the only one left who remembers the Infinite Earths. You see, I know the truth. I remember all that happened, and I'm not going to forget. Worlds lived, worlds died...."

-The Psycho Pirate

Introduction
Crisis on Infinite Earths is the grand-daddy of all crossovers. This major storyline was created in order to tidy up the major clusterfuck of DC Earths and to restrict it to a single one. These group of Earths were called the Multiverse and protected by the nigh-omnipotent Monitor, the storyline kicked off as the Monitor was defeated by his evil twin the Anti-Monitor and was sent into a coma.

The Anti-Monitor destroyed the different Earths as a means to make himself godlike, by the time the Monitor has awakened there was only a handful of Earths left from their original numbers. Doing a last ditch attempt to stop the Anti-Monitor, the Monitor gathered all the mightiest heroes of the different Earths and do battle with the Monitor.

Old school and classy, this is generally the starting point of DC Comics if you want to start at around the 90s.