"Superman" arguably is not a great movie. It's a good 1978 action movie starring Christopher Reeve as an alien with superpowers in a bright blue outfit. But it made more than $130 million in the U.S., and that's why it changed everything [source: IMDb].
There had been a few minor superhero movies before "Superman," but none of them achieved any significant success. "Superman" is, of course, based on the DC Comics character. And when Hollywood sees success, it tries to replicate it. Several more (less successful) Superman movies followed. In 1989, another DC hero, "Batman" made more than $400 million, and the era of the superhero movie really began [source: IMDb].
Superheroes dominate the action movie market today. "The Dark Knight," "The Avengers," "Spider-Man," "The Watchmen," "X-Men," "Iron Man," — all huge hit movies. Plenty of less successful or lesser-known superhero movies have been made, too. Remember "Tank Girl," "Daredevil" or "Green Lantern"? Superheroes are now a much bigger part of pop culture than they were in 1978, and it was the box office success of "Superman" that opened the door.