Properly speaking, you may say that a novel is 'comic' or 'tragic', among other possibilities, and describe it as 'a comic/tragic novel', depending on its content, but to call a novel either 'a comedy' or 'a tragedy' is improper (a category mistake), because the terms 'comedy' and 'tragedy' name dramatic genres subject to rigid formal conventions that a novel, by definition, cannot satisfy. Strictly speaking, 'a comedy' or 'a tragedy' must be literary works written in dialogic form to be exhaustively represented on stage. The terms 'comedy' or 'tragedy' are nevertheless sometimes used to describe comic or tragic CONTENT, respectively, but only as mass terms, without the article 'a'. As to the difference between the comic and the tragic, roughly speaking, the comic makes spectators laugh (and learn from the folly of their likes) whereas the tragic makes them feel horror at the fate of their betters and experience compassion, with a catarctic and purifying effect on them. You should read Aristotle's Poetics. It is just about thirty pages; read it through if you can. If you cannot, parts 3, 5, 6, 13, 14 and 18 deal specifically with your question as originally formulated more than two thousand years ago.