It started innocently enough. Someone on a forum I'm on just asked the question as to what the terms meant and what the difference was in the context of a discussion of a particular genre (horror in that instance) and while it does seem like a fairly simple thing to ask and explain, to do so actually requires a little bit of digging into the history of the art-form and strikes at a fundamental issue with it, i.e. that in spite of four decades of a counter cultural movement in its favour for recognition many still see it as an innately childish genre.
Its also a question I've seen asked and have personally answered many times before so this post to my own blog is essentially so I have the answer down in one place handy to send to the next person who asks.
The preferred term is sequential art. We're talking about a series of pictures, anything from two to potentially infinity, arranged in a specific order to express something, like to tell a story or invoke a feeling or idea. This might or might not also include text though usually does. The general public however have been calling this art form "comics" for more than a century. This comes from when news papers would have short 2-4 panel "comic strips" (so called because they were typically meant to be amusing diversions from the daily news, and aka The Funnies) alongside the text, which some enterprising publishers started putting into "comic books" to sell on their own without the accompanying news articles etc.
Although this came to encompass may other genre, popular ones being horror, war, crime, romance and of course Superheros, "comic books" still stuck for the format. In most of the english speaking world these were and remained popular from the 20s onwards. In the 1950s there was a bizarre moral panic in the United states about juvenile delinquency and the effects of these comic books on the fragile minds of the youth which led to congressional hearings and the industry shitting itself and self-censoring anything that wasn't explicitly for children (this would be repeated elsewhere in the anglosphere by Communist Party front organisations, and if that sounds like I'm crazy or making this up, I fucking wish). This killed off most of everything except superhero comics put out by the Big 2, who would become DC and Marvel who instituted the Comics Code, which committed them to not put out anything remotely adult or even YA.
This state of affairs would continue for decades (in the English speaking world, South America, Continental Europe and most notably Japan always had and continue to have thriving varied comics scenes), with some exceptions in the underground and indie press. What happened eventually, to really simplify things, was writers who came up through 2000AD in Britain from the counterculture, starting with Alan Moore would give the entire industry a huge shot in the arm and actually write for the big American publishers with a level of sophistication and a subversive ethos that was lacking. It was still superhero stuff but they were bringing the psychedelia and the punk attitude, and some technical innovations to the storytelling itself. This eventually got picked up by the mainstream media who coined the term "Graphic Novel" to differentiate stuff like Watchmen and The Dark Knight Returns from the kiddie shit, long soap operas with guys in pyjamas and capes beating each other up. It would also get applied to Maus and retroactively to previous attempts to do more adult work in the medium, like A Contract With God.
Also, in most mainstream book retailers something you'll see labelled and sold as Graphic Novels collections of works originally sold as individual issues of 24 page comic books where a complete story line or "arc" that tells a relatively self contained narrative is put into one volume. In the west these are more precisely referred to as Trade Paper Backs or TPBs, though that's an industry term and Graphic Novel or comic are as good as any. I only really include it here for the sake of completion. In Manga culture when you see the similar thing of serialised stories taken out of their respective periodicals (Manga tend to be sold in huge compilations, a bit like 2000AD but much larger, Shonen Jump and Animage being two important ones) some like to use the Japanese word Tankebon.
Personally I use sequential art as much as I can but only where appropriate, its the correct term but is really only known to those already acquainted. Most of the time though I'll use Comics as its what most people know. I don't like "Graphic Novel" except when specifically referring to something that has been written in a long format to tell a single contained story, i.e. like a novel, because there's an implied snobbery and hierarchy over mere "comics".
