Snuff is a genre of film that purportedly shows the murder of another person, usually a woman. Snuff porn is another genre of snuff film that includes sexual content, often of a violent nature.
Although there is pornography that advertises itself as "snuff," according to fact-checking site Snopes.com, snuff films are more urban legend than reality. Rumors of such films have circulated since the 1970s, but all those investigated have turned out to be fake. In other words, the "victims" in these films were found to be alive and well after the fact.
The term is believed to have been coined in connection with the Manson family murders, which took place in 1969.