When Marvel Comics publisher Martin Goodman ordered his writer/editor Stan Lee and artist/co-plotter Jack Kirby to create a superhero team book ("Fantastic Four") to cash in on the success of DC Comics' "Justice League of America" in 1961, none of the three men had any idea that their efforts would result in an incredibly influential line of comic books whose impact is still felt today.
Lee & Kirby, along with others later on, came up with the concept of superheroes with personal problems much like the ones readers themselves faced. "Spider-Man", created by Lee & Steve Ditko, was actually in the beginning a high school student unpopular with his peers and always broke. In addition, there was "Iron Man", a millionaire arms-maker with a bad heart; "The X-Men", a group of teenage mutants whose individual powers were feared by the general population, who thought them freaks; "Daredevil", a blind attorney with heightened super senses; and "The Hulk", in reality a meek scientist who, under times of stress (being called a "spineless milksop" by your girlfriend's dad will do that), would transform into a seven foot, perpetually angry, green skinned behemoth who lashed out at anyone. Lee and company also revived some of their classic characters from the forties and fifties and gave them problems too, like "Captain America" (who lost his young sidekick Bucky*in WWII and now doubted his calling in these complicated modern times), "The Human Torch" (now a temperamental teen-aged member of the Fantastic Four instead of the android** version in previous decades) and "The Sub Mariner", who was just plain mad at everybody today as he was in the old days. Add to the mix Lee and Ditko's "Dr. Strange" (an arrogant former surgeon who turned to magic after his shattered hands could no longer perform complicated surgery), "The Mighty Thor", "The Black Panther" (whose debut appearance in 1966 made him the first black superhero), "Nick Fury, Agent of SHIELD", and -stop giggling!- " The Astonishing Ant-Man", and the Marvel Age Of Comics was underway!
Readers responded to these flawed-but-still-heroic characters so favorably that Marvel's sales quickly soared and actually beat their competition. As a result, DC and other publishers tried copying Marvel's style, with variable degrees of success. One company, the now-defunct Charlton Comics, located in Derby, CT, even got Steve Ditko (who angrily quit Marvel in 1966 over artistic and financial differences) to help create their short-lived (1966-68) Marvel-like "Action Heroes" line with characters like "Captain Atom" and "The Blue Beetle". But Marvel never lost their edge, and they still continue all these decades later, albeit with new talent, to put out current adventures of their various characters in comics and graphic novels, and overseeing their properties in successful movie, TV, online and video game adaptations, while influencing other media at the same time. ("The Dark Knight", spotlighting DC's "Batman" character, was definitely molded by Marvel's established house style.)
Beginning in 1998, Marvel's current editorial overlords decided to reprint, in inexpensive 400-500-page phone book-like tomes, most of their past comics (in black and white, not color) from previous decades in an ongoing line referred to as "The Marvel Essentials". New readers and older fans can now read the earliest-known adventures of Spider-Man in the multi-volume "Essential Spider-Man" series, the Fantastic Four in "Essential Fantastic Four" and so on.
Greenwich Library carries Essential volumes of Fantastic Four, Spider-Man, the X-Men, Captain America, Daredevil and others, including later-day ones like "Iron Fist" and "The Punisher". If you're interested in the genesis of the "Marvel Universe", the Essentials are a good place to start.
(* In recent years, it's turned out that Bucky not only survived WWII but he's also, in the current Marvel continuity, the "new" Captain America. Steve Rogers, the original CA, was seemingly killed off, but then got better.)
(** Both the original android Human Torch and his (human) teen sidekick "Toro" were killed off by Marvel in 1966 and 1969 respectively, apparently to avoid confusion with the Fantastic Four version. The original Torch was revived in the late 80s but killed off a second time in 2004. Recently, both the original Torch AND Toro were revived (again in the Torch's case) and headlined their own eight-issue miniseries, "The Torch", set in the present day and which featured guest appearances by the Fantastic Four (with THEIR Human Torch), among others. Let's hope both characters stick around a little longer this time.)