Graphic Novel vs Comic Book: What's the Difference
The terminology
'Graphic novel' was popularised as a marketing term in the late 1970s and early 1980s, partly to distinguish longer works from monthly comics and partly to get them into bookshops rather than newsagents. Will Eisner used the term for A Contract with God in 1978; it gained wider usage when Watchmen was collected into a single volume.
In current usage, 'graphic novel' tends to mean either a single original work in comics form, or a collected edition of a serialised story. 'Comic book' tends to mean a monthly or bimonthly serialised publication. 'Comic' can mean either.
The snobbery
The distinction carries a cultural weight that is partly snobbery. Calling something a graphic novel implies it is for adults and deserves serious attention; calling something a comic implies it is for children and can be safely ignored. This is silly, since the quality of the work is in the work, not the format or the terminology.
Some of the most serious and accomplished works in comics are serialised rather than original graphic novels. Some graphic novels are trivial. The terminology does not predict quality.
Why it occasionally matters
The distinction has practical consequences in library cataloguing, bookshop placement, award eligibility and rights negotiations. Some awards accept only original graphic novels and not collected editions. Some bookshops stock graphic novels separately from comics.
For readers, the only question that matters is whether the work is good. The format question is secondary to whether the creative team had something to say and said it well.