Superman vs. Spider-man by CBMM1, on Flickr
Here is a little backstory on the comic via Wikipedia:
Superman and Spider-Man meet for the first time and must work together to prevent Luthor and Dr. Octopus's nefarious plans for world domination. This is the first DC/Marvel crossover ever published. DC and Marvel had previously collaborated on a treasury edition of The Wizard of Oz, but this is the first time characters owned by both companies were used together. It would be followed by several other crossovers in the years to come. In the introduction to Crossover Classics, which contains the story "The Battle of the Century!", Gerry Conway states "Purists may complain that we never explained how Superman and Spider-Man ended up in the same 'universe'...; to our minds, how they got there was beside the point."
Edited by CBMM1, 23 November 2012 - 01:26 AM.