Erotica
Erotica focuses heavily on the sexual relationships between the characters. While erotica often features a romance at the core of the novel, today's erotica also pushes boundaries including multiple partners (sometimes simultaneously) and kinky situations. Language can be graphic and sexual fantasies -- often not considered politically correct in today's romance novels are enacted.
As with all romance genres, erotica appeals to a wide readership base. For a variety of reasons, erotica is especially popular in e-book format. Ellora's Cave, who trademarked the term Romantica to describe the erotic works they publish, started out as a purely online publishing venture, but soon expanded into bookstores -- a switch from the traditional publishing model.
Most major publishing houses now feature erotica imprints. These include Kensington's Aprhodisia and Avon's Red imprints.