My mind wandered while I was trying to explain marketing categories, and how they’re not solely determined by subject matter, to someone in a comment thread at Tor.com:
Say your book features a strange and powerful device, the Transnistrian Infundibulator:I stopped there, but you don’t have to.
If the storyline is about the inception, interim difficulties, and eventual happy resolution of the relationship between the inventor of the Transnistrian Infundibulator and some nice young woman, it’s a romance.
If he’s a scholar studying the Transnistrian Infundibulator, she’s a governess, and his best fossil specimen of T. infundibulator falls out of his pocket during a reception at Almack’s, it’s a regency.
If one or both of them is not 100% human, they meet cute while fighting off spooky badguys, and the Transnistrian Infundibulator is an ancient magical artifact they use to defeat said badguys, it’s a paranormal romance.
If she’s his lab assistant, he thinks she looks hot in goggles and a tool belt, and the Transnistrian Infundibulator is a huge rivet-intensive steam-driven mechanical wombat, it’s steampunk.
If the Transnistrian Infundibulator is magic, but instead of working like a handheld appliance, it generates profound and numinous changes that affect the world as a whole, it’s probably fantasy.
If figuring out who killed the inventor of the Transnistrian Infundibulator involves complex railway schedules, an old doctoral dissertation, and the exact whereabouts of all the houseguests on the night it happened, it’s a mystery.
If all the elements in the preceding paragraph are present, but are mere background to an increasingly convoluted series of unfortunate encounters, improvised excuses, assumed identities, and inadequate hiding places, it’s comedy of manners.
If the Transnistrian Infundibulator is a very important box with blinky lights on it, but its only perceptible function is to motivate asst’d spies, gangsters, goons, private investigators, international men of mystery, and hot babes to steal it/recover it/put it out of commission/get hold of the plans for it/etc., it’s an action-packed mystery, or possibly a thriller. If characters who already know how the Transnistrian Infundibulator works stop the action dead in its tracks while they explain it to each other, it’s a technothriller.
If the effects of the Infundibulator leave blood and brains splashed on the walls, and the last surviving viewpoint character winds up in a desperate hand-to-hand fight in the dark with one of its half-rotting revenant Transnistrian victims, it’s horror.