I have never heard of a movie rated NC-17. What is it?

These movies feature extreme content, and are not usually shown in theaters. They might feature intense, realistic, and gratuitous violence, like Henry: Portrait of a Serial Killer, or they might feature lengthy scenes of explicit sex, such as Verhoeven's Turkish Delight. These films used to be called "X-rated" films, and, while there are some good ones (two are listed here), it is good to know what you're getting into before watching one. Treat these on an individual basis, and do your research before watching them, especially if you plan to watch one with others.