It's a pretty tired line to walk, but it is one people (read: nerds) still emphasize - are you a Marvel fan or a DC fan?
It's a stupid arbitrary line to toe - why would you shut yourself off from 35% of the entire comics market just to enjoy the other 35% of the comics market? Plus, there are only so many books out there - you're going to run out of shit to read at some point, so why limit yourself? There are amazing books by Marvel, DC, Vertigo, Image, Wildstorm, IDW, Boom, Archie - read all the cool shit you can, man. You're going to die someday, so why die on such an arbitrary fucking hill now?
That said, there's a world of difference between the two big publishers, that it's easy to see why lines would be drawn in the sand, as stupid as they are. So let's take a little dive into just what makes the two so different, and occasionally - so similar.
Also, this is gonna be a long and vague one, so - strap in, nerds. Get ready for bramblin' Captain Rambles in 3, 2... 1.
DC
Don't read any favoritism on me listing DC first, they just happened to establish themselves first, more or less, and I once read somewhere that timeliness is next to godliness, or something like that. Keep in mind I'm not a historian. That sounds awful. But I'm going to lay some groundwork, so please tell history to watch its feet - I'd hate to step on them.
DC put itself on the map back in 1934, but back then it was called National Allied Publications and it put out, more or less, bulk tabloid size comic collections not that different then what you see in newspapers - stuff like Beetle Bailey and Garfield. They all had a humor slant that would eventually be replaced by the youth fascination with superheroes. 1937 would see Detective Comics, their next big breakthrough comic, but it would be released from what was essentially an offshoot company. Then they'd publish Action Comics 1 in 1938, and the world would meet Superman. Detective Comics would hit issue 27 in 1939, and the world would get Batman. Corporate consolidation would happen, we'd get National Comics, and then when it became clear that Detective Comics was their best selling book, a final name change to DC would basically give us the beginning of what was called the Golden Age of Comics. in 1941, Sensation Comics would debut, and then the world had Wonder Woman. DC's trinity would be complete.
Shit was going super well during the war - turns out seeing people have superhuman abilities and unrelenting moral character was a good thing in one of humanity's darkest periods. Eventually we'd get the Flash, Green Lantern, and a few other names people today know - ish.
What stopped these superheroes from running into Germany and beating the shit out of Hitler to end the war yesterday? Well, it turns out there's a very good reason - Occult-obsessed Hitler found the Spear of Destiny - y'know the spear a roman guard used to pierce the side of Christ, and it gave him magical powers. Notably, he could mind control superheroes who were within his border. So that relegated most heroes to stateside operations and... yeah, it's pretty sweet that they thought of a reason though, right? They banded together and decided to make the best of a bad situation and form a team. Known as the Justice Society, Wonder Woman, Batman, and Superman were all super busy in their own books, so they were honorary members. The Flash, Green Lantern
By the late 40's/50's, the well on Superheroes had kinda dried up. DC's big three - Batman, Superman, and Wonder Woman - would keep seeing print, but every other superhero would ride out into the sunset with little fan resistance. These were funny books after all, not gospel, so there wasn't exactly a revolt or anything. Thank god Change.Org wasn't a thing then, though there probably wouldn't have been a fight.
Superheroes faded, and romance, science fiction, western, and every other genre under the sun that didn't involve tights came into prominence, feeding imaginations the world over.
That last part's probably not true. It's probably more like the country over. But, like - more than the tri-state area, less than the world, so pick your own happy medium there.
Y'know how nowadays, every so often there's a news story about how video games are corrupting our youth, making them more violent and terrible? In the 1954 there was a version of that for comics. Fredric Wertham's Seduction of the Innocent painted comics, specifically superhero books, as the nation's scapegoat, saying they were making kids worse and generally shitty, so that helped push in the new paradigm of everything but superheroes.
But then in the 1956 DC realized that dude was kind of a douche and decided to reintroduce its superhero lines. The Flash was brought back, but with a twist. He was a new character with ostensibly the same powers, and same with Green Lantern - these were basically modernized takes on the same characters from the 30's/40's, updated to reflect the times just a little bit better. Green Lantern's ring was no longer magic-based, it was science based, and the dude wielding it was a test pilot. Barry Allen was the Flash now and instead of being a football star science student, he was a forensic scientist working for the police department. These were significantly more grounded stories because they actually gave a shit about the characters backstory beyond the "hijinks" that could ensue when they weren't wearing their masks.
Before long, they all started hanging out. Before long - they had the Justice League. And the DC Universe as y'all know it is more or less born. And then destroyed. And then reborn. Let that happen a few more times and all of a sudden it's 2018 and you're basically all caught up.
Marvel
In 1939, Marvel had another name - Timely Publications. Their first book, however - Marvel Comics and wait...
God, comics are so stupid, I love them.
Marvel Comics saw the debut of The Human Torch and Namor, The Sub Mariner. By 1941, we'd get Captain America courtesy of Joe Simon and Jack "The King" Kirby. Those dudes teamed up and rocked the Axis powers as best they could as a team called The Invaders. And they'd wreck house. And be vaguely racist. Marvel doesn't want you to remember that part now, but hey, it's there.
Timely Publications became Timely Comics, but unfortunately the name change didn't make them any more immune to the superhero bubble bursting than DC was. They toed the line with what was popular at the time, chasing trends like it was their goddamn job, because, well... I mean, like, it was.
Sorry, that one got away from me a bit.
In the 1950's they were known as Atlas Comics, and Stan Lee, the godfather of Marvel Comics himself has even stated that the only reason they survived was because they were fast and good enough. They'd turd out Romance, Horror, Western, and every other thing under the sun and do it Moneyball style.
But by 1961 when DC had reintroduced superhero comics and shown they were here to stay, and after Atlas had a name change, the newly renamed Marvel Comics finally had a new trend to chase. In 1961 The Fantastic Four debuted and rocked everyone's socks off, because their target demo wasn't kids the way most comics were with other publishers. Marvel wanted an older, smarter audience and created more mature, focused books that were a bit more complex and challenging.
After a big hit like that, Marvel had a universe to fill around it. 1962 saw the debuts of Spider-man, Thor, Ant-Man, and the Incredible Hulk. The following year the X-Men, Iron Man, and Wasp made their first appearances. The next year, Captain America reemerged in issue 4 of Avengers after taking a break since the end of the war, and was explained into current Marvel continuity. Daredevil would see his first issue in '64 as well and y'know what? I'm gonna stop there. There are too many damn heroes for me to tell you when they all came out. Shit like that is what wikipedia is for.
But you can see some common ground, right? These two mediums were basically built from the ground up to chase trends cheaply, and then you can tell that they found their own voice pretty quickly - not just reflecting the times but reflecting them well. Little bubbles of time trapped forever.
What separate them? Next time, champions.