In 1976, Superman vs. The Amazing Spider-Man touted "The Battle of the Century" between these two superheros.
In the comics, the two don't meet each other through alternate universe shenanigans or anything (as was the case with DC vs. Marvel). Instead, the two apparently just bump into each other when they travel to a convention:
Superman is owned by DC Comics and Spider-Man is owned by Marvel, and to my knowledge, the two don't exist in the same universe (e.g. when Hulk goes on a rampage, Superman doesn't come to save the day).
How is it that the two were able to meet if they exist in separate universes? Is this in alternate continuity, and just not revealed to the reader?