While putting pictures next to words is not new, comics, for all intents and purposes, are a modern invention. This guide is intended to teach the complete outsider the history of the medium, from start to finish.
The Comic Book's Ancestors
Cartoons, or humorous pictures that usually appear in magazines or newspapers, have been around since at least the the 1700s. They finally gained the name cartoon, after an earlier form of cartoon. Artists would often sketch out their paintings, and these sketches were known as cartoons. Eventually, the term came to apply to single panel satirical works, especially political cartoons, and eventually, gag cartoons such as the work of Gary Larson.
Eventually, cartoons were strung together to tell a story, and these became known as comic strips. The first real comic strip was Hogan's Alley by Richard F. Outcalt, which came to star a bizarre child known as The Yellow Kid. This form would eventually become a staple of newspapers, usually taking up their own page or pages, even today. Eventually, the form would make the jump to the Internet, becoming webcomics. Arguably the first was T.H.E. Fox (a.k.a. Thaddeus) by Joe Ekaitis.
The Golden Age
Just as comic strips began as cartoons that ran together to tell a longer story, the first comic books started out as comic strips published together. Eventually, this trend would grow into the graphic novel, when monthly comic books were eventually published together. The first comic books were published in 1934.
Four years later, a new type of comic book was published. National Allied Publications published the still ongoing series Action Comics #1, which introduced Superman to the world. Eventually, the superhero genre would come to be more or less synonymous with comics, but at the time, it was something fairly new. Many other classic characters would follow, including Captain America and his Hitler punching ways, and the Batman.
The Silver Age
Alas, the Golden Age didn't last. People just didn't really want superheroes at this time, so American comics became, for the first and possibly last time, diverse . Ironically, the saviour that would bring superheroes back was one Frederic Wertham, who published the book Seduction of the Innocent, in which he accused comics of just about everything. The ultra-restrictive Comics Code of the early 50s killed off popular crime and horror comics, leaving the superheroes to pick up the pieces.
In any event, superheroes were okay, as long as they were a bit ridiculous. And they were a bit ridiculous. Still, the modern Marvel universe was born during this time, with DC becoming it's chief rival. To this day, DC and Marvel are the Coke and Pepsi of comic book publishers.
The Bronze Age
The birth of the seventies brought with them a less goofy group of comic book heroes. Though they wouldn't get quite as serious as the later ages of comic book superheroics, this is when the old Comics Code bit the dust.
Stories began to be about issues of the day. The thing that finally killed off the Comics Code, however, was the drugs. Admittedly, both Marvel and DC's stories involving drugs were very anti-drug; they were meant to keep kids off drugs. Still, they violated the Code, so publishers finally decided to just go ahead and go without Comics Code approval, and it worked. Comic book shops still carried the titles, and the message was spread. This was also about the time the X-Men began to really represent oppressed minorities by actually including them on the team.
The Dark Age
The recent comic book series Wanted by Mark Millar (which sort of inspired a movie with Angelina Jolie) features a world where there are no superheroes, because the supervillains won. In this world, 1986 is the year the superheroes died.
1986 is the year of Alan Moore's Watchmen and Frank Miller's The Dark Knight Returns. Shortly thereafter, the DC universe was available only in shades of black and red, while everyone in the Marvel universe carried an extremely large gun. Characters like Hellboy and Deadpool were born during this era; they're the nice ones.
Essentially, the 90s were like the 60s; completely ridiculous. Except they were also a lot less fun.
The Post-Modern Age
After all that doom and gloom, it may be surprising to find out "apocalypse" doesn't follow the prefix post in the final age's name. Though comic books have been struggling for legitimacy since their inception, they finally found a modicum of success with the Post-Modern movement, which is also the only art movement that routinely uses comic books as inspiration.
In a nutshell, ever since the turn of the century, comics have become increasingly self-aware, and increasingly weirder. Both the bizarre adventures of the 60s and the excesses of the 90s, which could easily cause embarassment, are instead mined for story ideas, though now in a much more self aware veing. Most Post-Modern comic book writers are aware of the inherent ridiculousness of older storylines. They wouldn't have it any other way.