At a very basic level, a reverse harem romance is about one woman in concurrent relationships with three or more men. At the end of the story, rather than choose one among them, she chooses to continue having relationships with all of them.
Other than that basic premise--encoded in the RH motto "Why Choose?"--a reverse harem may belong to any subgenre of romance. A reverse harem novel may be paranormal, contemporary, science fiction, young adult, adult... as you can see in the variety of reverse harem books I've recently released. :)
In Rift Spun, a novelette in Night Fire, arachno-mage Rhiann plans to use the magic of the Rift to take down the ruling Archmage Prime, a necromancer who killed her family. To do that, she’ll need to find three men willing to become her familiars—one shifter, one demon, and one angel. Lucky for her, she won’t have to choose only one.
Tell me: what kind of reverse harem do you think would appeal to you most?