American comic book tropes are common elements and literary devices related to American comic books.
Continuity
Comics continuity almost-always refers to the existence and use of a shared universe, although any comic can have internal continuity independent of this. Simply, the term describes a consistency of internal plot, and usually of characterisation and external references also. Initially, many comics were stand alone, "done in one" stories with a beginning and end taking place within the confines of a single comic issue, often structured in chapters as are most novels. Over time, the comics companies realised the lucrative potential of the crossover comic, whereby other characters from a company's shared universe appeared in issues of each other's comics. This ultimately led to the formation of "team" books such as the Justice Society of America, Justice League of America and Avengers.
During these crossover character interactions, editorial footnotes would often reference previous adventures and comics issues, but an actual editorially enforced "continuity" was not strictly adhered to, leading to some characters' actions appearing "out of character", or outrightly contradicting earlier plot-points. As comics were deemed largely ephemeral items, this was not considered that much of a problem, until the full advent of comics fandom. As a result of fan/reader scrutiny, the continuity both of individual characters and of the wider universes in which comics companies' characters interacted began to become more important. The Marvel "No Prize" became a humorous method by which readers could write letters to authors and editors pointing out mistakes or "continuity errors" in various comics, and were then named in print and awarded a "No Prize" (in reality a coveted sheet of paper declaring itself a non prize).
In 1985, cross-universe continuity took on new levels of depth and (intended) consistency at the two main comics companies: DC and Marvel. Marvel launched its cross-line toy-driven-event Secret Wars, which required all characters to undergo specific changes at specific times, and required considerable editorial dictates and conformity. DC launched the Crisis on Infinite Earths, one of the earliest maxi-series', to address universe-wide continuity and attempt to explain away, remove or revise all previous errors in continuity. The reader was reminded that the DC Multiverse consisted not merely of the core DC Universe, but of a number of different iterations of various heroes on a multitude of different planets. Companies and characters purchased by DC (such as the Charlton Comics characters and Captain Marvel) as well as older characters like the JSA were (re-)assigned their own Earths, which were then destroyed and folded into one, core Earth. This naturally resulted in a number of contradictions and discrepancies in individual characters' histories, so a new, uniform continuity was created and the revised origins of the resulting heroes were retold in the hopes of maintaining consistent continuity.
With hundreds of characters and dozens of writers, over the years uniform and consistent continuity is difficult to maintain, and most comics companies periodically address the erosion of internal consistency with big "events" designed to explain and simplify (although at times they do neither) discrepancies, and maintain continuity.
Canon
Similar to internal continuity, the canon of comics characters/universes is often subject to change, but refers to the stories which are, at any one point, part of the "official", "accepted" history and story of particular characters/universes. Alternate versions of characters (such as DC's Elseworlds and Marvel's speculative What if...? titles) are necessarily not canon, but the stories can change from being non-canonical to being accepted as canon - and vice versa. In particular, line-wide continuity-changing events (such as DC's Crises and Marvel's controversial recent Spider-Man: One More Day storyline) retroactively affect which stories are part of a character/universe's core canon, as they may revise or ignore previous events and happenings.
For example, DC's Crisis on Infinite Earths addressed continuity and consistency errors over almost 50 years of comics publication, and retrofitted events and characters into the history of the DCU as if they had always been there (for example, the JSA went from being JLA-contemporaries from a parallel world to being their earlier, historical counterparts some years previously). The Post-Crisis DC Universe removed many stories from "official canon", explaining them as Imaginary Tales or ignoring them completely.