The earliest comic books were compilations of previously-published humorous strips from newspapers. The first comic book was Funnies on Parade, published in 1933. Comic books soon diversified to include stories from a variety of genres, but the name remained.
Collectors categorize comic books by various "Ages." The publication of Action #1 (June, 1938), which contained all-new material and was the first appearance of Superman, marks the beginning of the Golden Age (1938-1956). Superman was soon followed by other super-characters. At their height, superhero comics were selling up to a million copies per monthly issue. The 1940s and 1950s also produced many non-superhero comics.
After the end of WWII superheroes declined in popularity, and the comic book industry struggled. Fortunately, new genres reflecting trends in popular culture -- crime, horror, science fiction, and romance -- emerged. The popularity of these genres gave the flagging comic industry a boost, but was almost it's [sic] downfall as well. In 1954 Dr. Frederic Wertham published "Seduction of the Innovent," which blamed comic books for juvenile delinquency. There were comic book burnings and Supreme Court hearings; to protect their industry comic book publishers created the Comics Code Authority to regulate and curb violence in comics. The Code symbol subsequently appeared on approved comic books, curtailing the crime and horror genre.
The Silver Age (1956-1970) saw superheroes make a comeback in updated forms. It began with the publication of DC Comics' Showcase #4 (Oct. 1956), which introduced the modern version of the Flash. Following the success of Showcase #4, several other 1940s superheroes were reworked. The characters' names remained the same, but their costumes and identities changed.
During the Bronze Age (1970-1985), traditional superhero titles remained the driving force of the industry. However, darker plot elements and more storylines addressing real-world issues (racism, drug use, urban property, and environmental pollution) began to flourish during this period.
The Dark Age -- also known as the Iron Age -- began in 1986, with the publication of Frank Miller's The Dark Knight Returns and Alan Moore's Watchmen. Dark and gritty , they kick-started a trend for portraying superheroes as Anti Heroes.
The Modern Age (1996-present) has some lingering elements of the Dark Age, but also a return towards lighter and happier comics. Other features of the era are the rise fo the graphic novel as respectable reading material and non-superhero comics becoming more mainstream.