Strictly speaking, a graphic novel is a medium where the story told in the form of drawn pictures, similar to a comic book. It isn't an illustrated book where you have prose and pictures thrown in. The stories is told entirely in pictures with word bubbles and text boxes.
It can refer to a collection of individual comics. The individual issues are usually collected where each collection follows a particular story arc, so that when picking up a volume there is a full story within. The Sandman by Neil Gaiman and The Watchmen by Alan Moore are examples of these, where the stories actually first appeared as individual comic books. Majority of graphic novels that people speak of refer to these, albeit it isn't entirely accurate.
The term trade paperback also refer to collections of comics in a single volume, but it's, well, in softcover.
The term graphic novel can also refer to a work done exclusively as a full story in itself. These are the 'pure' graphic novels, for example Neil Gaiman's Mr Punch, or Endless Nights.
I always say to those who are looking for something new, but don't want to read superhero stories - pick up a good graphic novel or trade paperback collection (there are plenty of people here who would point you to the right material) and immerse yourself in a medium where the stories can be as interesting and as stimulating as the novels you read. And more colorful too.
Then come here and discuss.
ds
'Dont worry dear, its not a comic, its a graphic novel!!