He is mostly known for founding Nine Inch Nails. Most of the music he's done in the studio is done alone except for drums. When he does live shows, he hires musicians.
He's also been apart of other bands in the past, and on top of that, has worked with id Software on games like Quake and Doom 3, although the sound work he did wasn't included in the final release. It is available in Enhanced Doom 3, a mod for the game.
For more, check here: http://en.wikipedia.org/wiki/Trent_Reznor