There are numerous technical differences between Graphic Novels and Comics. The basic one would be that Graphic Novels are collected, self-contained stories, whereas comics could generally better be described as long sequences of linked episodes or single strips forming a generally ongoing narrative, if that makes any sense. There's a qualitative difference between, say Watchmen, and Spiderman, in the format and scope of the whole thing. There's also a difference between something like 'The Dark Knight Returns' and the Batman newspaper strips. It's also about the way you cut up the narrative of course, but only really in the final presentation. Most graphic novels originally ran as comics. In practicality, it's more a trade term: graphic novels and trade paperbacks are pretty much the same thing.
As for comics and manga, well, different presentation format (now, thank the gods, they've started selling un-flopped manga in english), different visual language (ZZZ versus nose bubbles, different way of approaching panels), largely though not exclusively based in different genres (superheroes and funny animals versus mecha and magical girls), rooted in different artistic traditions (Hogarth versus Hiroshige) though there is crossover both past and present. It makes commercial, as well as artistic sense, to present them as different things. Maybe not so much nowadays, as the two have blurred conspicuously (particularly western comics adopting concepts and aesthetics from manga).
Also, although the term 'graphic novels' might be seen as pretentious, I believe it has been crucial in gaining what literary acceptance exists of the medium. I don't think Maus would have won a Pulitzer if it had been presented as a comic book rather than a graphic novel. The term 'comics' just carries too much baggage from the dark years of the Comics Code. Hell, even back in the old days, underground comics used to call themselves 'comix' to distance themselves from that association. Maybe it's time to claim back the name for the medium, but I don't think so quite yet. Huge inroads have been made in to accepting sequential illustration (now there's a nicely pretentious term) as a legitimate, mature art form. I don't think we're ready just to go back to calling everything comics just yet.