Comics have been around for a long, long time. More than a hundred years in some forms. Initially devised as a way to collect the daily strips into a single, sellable format, it was the release of Action Comics #1 in 1938 that really brought them around and superheroes became the dominant form of comic book attraction. This was the start of the Golden Age of Comics. Superman, the star of Action, was followed by many more heroes, many of whom came from the science fiction tradition.
These heroes were huge, important, and always with a backstory that would inform everything about them as characters. Most comics at this point were not single character books, instead being a compilation of shorter stories put into a single book. Comics like All-American, Sensational, Whiz Comics, and Detective Comics all had stable of recurring characters. Superman became the first superhero to become the title character of his own comic, and many more would follow.
World War II made the war comic popular, and though superheroics never stopped being popular, by the 1950s, they were playing second fiddle to genre comics from companies like EC. Horror, SciFi, Romance, and Western comics were big sellers in that period.
In the mid-1950s, Julie Schwartz at DC came up with an idea. He re-launched the superhero comic with new ideas, and as one of the founders of science fiction fandom, and an agent of science fiction authors, he often pointed characters towards science fiction origins, characters, and scenarios. Characters were re-invented, and many new ones appeared, and some existing characters were teamed together, starting with Justice League of America, who defined the Silver Age of Comics. Stan Lee took many of these same ideas and began to work with them at Marvel, starting a two-party system that would dominated comics for three decades.
The Silver Age concepts today may seem somewhat silly, but they reinvigorated superheroes, and created many of the tropes we understand in comics through to today. Things like team titles, cross-overs, multiple timelines within a single company's titles, the "Imaginary story" (also called "What if…", and especially the idea of single hero titles. This was the period when many of the anthology titles finally died off, and the use of a single hero across several titles was typical.
The series here represent the peak of science fiction in comics. Many series date back to the earliest days of comics, when even the term science fiction was new. Many have run decades, with reinventions, new teams of writers and artists, and even complete re-inventions. At one point or another, most of these series have been touched by the two hands that defined the Silver best – Julie Schwartz and Stan Lee!