Sure, comics existed in some for or other before 1938, but it was Action Comics #1, cover-dated June of '38, with the image of people freaking out to Superman lifting a car over his head--and these were old-timey cars, possibly clocking in at several tons--that almost overnight single-handedly changed the nature of comic books. Super heroes were the new cash cow, and the machiavellian nature of early 20th-century printing turned to producing more and more of them with near-sweatshop precision. Sure, westerns and sports and pirates and horror and romance were still around, but superheroes dominated. Stories of this age, especially super hero ones, were based almost exclusively on their pulp ancestors, featuring manly he-men saving some skimpily attired woman who may or may not be able to handle herself otherwise but is usually just there for the arm candy. They often featured brutally evil villains, but that's okay, because they were opposed by often brutally heroic protagonists. Strangely, these dark stories were also often incredibly naive; sexually, politically, ethically, artistically. However, there were still bits of maturity inherent in these comics. There was one story, for instance, in which Clark Kent was investigating unsafe mining conditions, a bold stance to take at the time. Of course, it ended with Superman trapping the corrupt mine owner in a collapse. Like I said, brutally heroic. While super heroes were often criticized as fascists, it's interesting to note that most super heroes lacked one crucial factor that defines fascism: they weren't all that nationalistic at first. Sure, they'd defend America if it needed help, but it often felt that they did so because that's where all their stuff was--many super heroes were actually champions of the underdog, exemplified by the common American working man; since the American government at the time wasn't exceptionally interested in helping the common American working man, that meant they were as often counter-culture heroes as they were maintainers of the mainstream. This of course changed the closer to WWII America got, reaching a high point during 1941-1945 (the war years). Super heroes bashed (often racially) the Axis powers on their covers, often imploring the reader to buy war bonds and support war bills (or at least, to tell their parents to do so). This is interesting, and I'll get to why in a minute.
First, two other features of the Golden Age bear mentioning: there was much more focus on supernatural and magical themes and elements, and not always as villainous (in fact, usually a magical villain was countered by a magical hero, and some magical heroes had little to no supernatural enemies). Often, enemies were either typical old-school gangster types, the kind you'd likely to see in sporting tommy guns and wearing double-breasted suits; or else they were aging mad scientists. A theme that science was dangerous was much more likely than one about the "evils" of magic (this would change in the next age for at least two reasons).
But back to talk of the war. In some ways, the War Years were actually the true Golden Age of Comics: after the war ended, super hero comics sort of lost their appeal, as America sort of settled into a position as a global power (which they never ever were before, in any capacity--most Europeans viewed American help during the first World War as a lucky fluke. WWII changed that pretty quick) and tried to play their success in wartime into proving they were somehow pillars of moral purity. This sort of indirectly led to the near collapse of the comics industry, but before we get to that: super heroes were help up against the accomplishments ordinary and non-fictional men (and women) made in WWII, and the non-fiction heroes won. Super heroes almost completely vanished, aside from Superman, Batman, and Wonder Woman (the latter mainly because of a deal the creator had made that allowed the rights to the character to revert to him and his estate if a certain number of issues per year weren't published--this eventually was renegotiated--not because of any quality of story-telling; not that Superman or Batman could get by with any claims of superiority on that front), the magical-based (and original) Captain Marvel--who was during this time actually more popular than Superman, and so a target of a copyright infringement lawsuit that they eventually lost--and a few occasional attempts to wring the last bit of goodwill people had for super heroes--most notably, the Martian Manhunter, who debuted in 1955, just after the Golden Age ended.
Why did the Golden Age end? Well, a certain psychiatrist named Fredric Wertham published a book he'd been working on for
The book was a strong piece of sensationalism, and it worked wonders. People were terrified that little Jimmy might turn into a degenerate simply by reading comics, and Congress held hearings, and threatened to shut down all comics publishers. They managed to survive by creating their own, exceptionally restrictive policing agency called the Comics Code Authority, that made some interesting and strange choices in censorship. Depictions of homosexuals were banned, as was drug use or questioning of authority. Many monster and horror cliches were also banned (because it seemed that Wertham had a particular hate-on for horror comics), so for decades, comic books couldn't feature fights against vampires, zombies, or werewolfs (interesting factoid: writer Marv Wolfman almost didn't get credited for his first published story--about a wolfman--because of restrictions outlined in the Comics Code). While comics had to be put through even more obsessive and foregone conclusion-type investigation, they were allowed to live, but the damage was done. It looked like super heroes had just been a passing fad, and would never show up on newstands again. Comic publishers limped on with romance comics, licensed comics starring celebrities, humor strips, funny animals, westerns, sci-fi, even horror stories (tame horror stories, to be sure) . . . practically anything but super heroes.
However, it was because of this shifting away from super heroes and to other genres that would quickly lead to a resurgence of super hero (and comic book) popularity).
Aaron "The Mad Whitaker" Bourque; it's important to note that Wertham was aiming for a rating system for comics, and was completely dissatisfied with the Comics Code. Apparently he wasn't interesting in obliterating "mature reader" comics, just restricting the chances of impressionable minds reading them. In that respect, he failed spectacularly. Perhaps if he had done a more thorough job on his research, hadn't resorted to the worst of yellow journalism tactics, and been more moderate in general in his approach, comic books wouldn't have been relegated to just "kiddie books" until the late 70s in the eyes of most of the country. Or perhaps he just said all that after the fact. His book certainly painted him in a different light than later statements would.