At its essence, I believe our definition of the actual comic form has changed very little over time. Whether dime novels or pulp fictions or comic books and strips, the image that comes to mind is a story, typically told through both text and picture, that depicts a hero struggling against the greatest fears of contemporary society. While artistic styles, color technology, and even the icons used to depict these stories may change over time, the iconic format of the comic itself remains the same. What changes is the message portrayed though the story. This message always speaks to what society's greatest fear is, as it is this fear that the hero must struggle against and, inevitably, overcome. In America, the comic form has a history of presenting the national ideal of an individual and upholding what are viewed as societal values and identities. These then overcome any perceived threats to this national self-image, threats which change depending on the time.
In terms of content, the comic form was first found among the dime novels, the country was undergoing rapid urbanization in the post-Civil War era, and the western frontier was nearly, if not already, extinct. The sense of American identity, which to that point had been inseparable from "Manifest Destiny", underwent an identity crisis. Along with this closing of the frontier came urbanization and a massive influx of immigration. Many people feared that these things would combine to degrade civilization and the American way of life. In answer to this was the dime novels of popular culture, which often featured men who were the best of what society had to offer overcoming adverse conditions or enemies to behave in a way representative of American virtues. When the Great Depression introduced into the common mind the idea that living according to these virtues might not be enough to succeed for normal humans, superheroes were introduced as the more-than-ordinary individuals required to overcome the days adversity. Thus, the identity of the modern comic was born.
While the evolution of how the content of comics formed plays a key role in the view of comics today, the development of the physical format is equally important. Comic books emerged from the hybridization of comic strips and pulp fiction. While pulp fiction may have provided the basis for the fantastical stories, the comic strips are what gave rise to the combination of text and illustrations in sequential panels to tell a tale. Comic strips originated as weekly installments used to attract readers. Then they began to be issued in a compilation book format. Eventually, publishers began seeking new materials to sell in the books, not just reprints of old comics. Taking the idea material from pulp fiction, they applied to formatting of the comic strips. In this way, comic books in their familiar format developed.