Canon: What is it?
The quick and dirty answer to the idea of media-based canon is this: Whatever the original source of a particular medium's stories are created, those medium become what is considered canon. In the case of comics, Superman events as portrayed in the primary universe of the production company of DC Comics would be considered canon. If Superman were to appear in other mediums such as cartoons, television or movies, those might be considered canon, if they conformed directly to what has been established in the comic medium. That is not always the case, and as such are listed as non-canon.
The question of canon becomes murkier in the case of movie properties that become other media such as Star Wars. The six original movies are considered canon, while the cartoon adventures of the Clone Wars are not. Dozens of books were made on the Star Wars Universe, but very few of them have been sanctioned as canon by Lucasfilms. This does not mean they are not good stories. This does not mean they do not expand the universe in ways that make the Star Wars Universe richer, better and more interesting. What it means, is that any changes that are taking place in that book are unique to that writer's interpretation of the material and may not be reflected in other writers work, or in the original source material.
Why such difficulties with canon, and why does it matter? Mostly, it is a style and creator-driven issue. When properties migrate from their primary source, information is either gained or lost. Such information may come from canon sources, or canon archives such as producer's notes or publications written for the original author's/creator's notes, but often is altered to make the new media more viable or allow different things to be done. Such transformation can change the core work significantly and this may offend purists of the original work.
Does canon matter to the common consumer of media? Not usually, they do not know the material well enough to notice a difference. Canon matters mostly to enthusiasts of the material who know the difference between a phaser and a phase converter.
Are the various Superman/Batman/Green Lantern/Other DC one off movies considered canon?
The best answer is that movies and some characters based in or on the DC Universe are not as likely to be canon, particularly in light of the recent soft reboot of the DC Universe, commonly called the DCnU or the New 52.
With the recent reboot, canon for almost all of the DC Universe has once again been shifted, for some characters only slightly, Batman for instance, still appears to be the same character from the previous DCU. Other characters such as Superman have experienced significant change, in appearance, demeanor, supporting cast and relationships.
These transformations will have drastic effects on the appearance and behavior of classic characters from the previous DCU such as Oracle, who as Barbara Gordon has returned to the cowl as Batgirl. The fate of the previous Batgirls (Cassandra Cain and Stephanie Brown) has not been determined. The next five years will likely change almost everything we know about the DC Universe, and by DC's standards, hopefully for the better. DC is counting on this transformation of their universe to inspire a new generation of readers unfamiliar with the characters beyond cursory information.
The old canon, however, would still not have supported most of the movies created in recent years by DC, particularly along their animated lines. Writers took significant liberties for the sake of transferring the stories to a movie format. One of the primary considerations is how to transfer years of history and comic stories into a two hour movie, no easy feat. So much of the comic canon is tossed out the window for the sake of story continuity, speed of story development, pacing and movie-goer's attention spans.
I would not worry as much about movies being canon and instead hope they are good enough to keep viewers attentions and perhaps drive interest in stories enough to keep comics coming out. Canon is a moving topic in the comic industry and you have to consider each age of comics as canon often only with itself and the generation written around it.
- The New 52: An overview of the DCnU line of comics and what this means to the DC Universe. [Stack Exchange Blog Post]
- How Much of DC Heroes' History is Lost With 'The New 52'?: A recent article in Comicvine discusses exactly how much canon has been lost or replaced. [comicvine.com]