NIN's sound has variously been described as alternative, electronica, heavy metal, rock, synth pop, or, most commonly, industrial or industrial rock. Regarding his music being categorized as industrial, Reznor said in a 1994 Axcess magazine interview:

"What was originally called industrial music was about 20 years ago Throbbing Gristle and Test Dept. We have very little to do with it other than there is noise in my music and there is noise in theirs. I'm working in the context of a pop song structure whereas those bands didn't. And because someone didn't come up with a new name that separates those two somewhat unrelated genres, it tends to irritate all the old school fans waving their flags of alternativeness and obscurity. So, I'd say I've borrowed from certain styles and bands like that."

NIN's songs cover a range of genres: "The Perfect Drug" has the flavor of drum and bass, "Down in It" influenced by early Skinny Puppy, "Happiness in Slavery" is tinged with industrial metal in the vein of Ministry, "The Frail" is a melancholy piano piece, and most of Pretty Hate Machine could be considered dark electronic pop in the style of Depeche Mode.

courtesy of wikipedia.org
http://en.wikipedia.org/wiki/Nine_inch_nails