Technically, there's no such thing, since technically, they're just the males of the Lilu, not a species unto themselves. Succubi don't technically exist, either; they're female Lilu. But since the two genders are different enough to be treated as members of different species, it seems best to go ahead and act like that's true.
Incubi look human, for the most part. They also look really, really good, like the hottest men ever to walk the planet. The trouble is, most people can't agree on what they looked like after they leave the room. That's because when you combine hormones, pheromones, and supernatural sexuality, looks cease to matter quite as much. This can be awfully hard on the nerves of the poor incubi, who either become incurable shut-ins, or spend their time seducing people for fun. The first variety are rarely an issue. The second, on the other hand...
Well. There's a reason the Covenant used to prioritize incubus hunts over anything short of dragon-slaying.
Aunt Jane married an incubus. They have two children, Elsinore and Arthur, who are remarkably well-adjusted, all things considered. Neither of them dates much.