At one point they essentially sued Fawcett Comics out of existence, going after the popular Captain Marvel, aka SHAZAM!, who they claimed was a rip-off of Superman. There are no superheroes without DC, and for decades they weathered any and all competition - in fact, they absorbed some of them.
They reinvented superheroes with Showcase #4 in 1956, when they introduced a new version of the 1940s character The Flash and kickstarted The Silver Age of Comics. They were called Detective Comics, Inc, later to be called National Publications, later to be called DC, and they invented superheroes with Action Comics #1. That's the most important thing to know, that DC was there first.
