It's a series of books by Jacqueline Carey (http://www.jacquelinecarey.com/books.htm). The female protagonist of the first three books in the universe (Kushiel's Dart, Kushiel's Chosen, and Kushiel's Avatar) is a courtesan who is also very much a masochist. There are different houses that train adepts who eventually bear the marques of their house as a tattoo all over their backs. That's a rather simplified description though. I'd highly recommend it for someone who likes both BDSM and fantasy.