The Nazgûl (which in the Black Speech means Ringwraith) are fictional characters created by J.R.R. Tolkien which appear in The Lord of the Rings series of novels. The Nazgûl were once mortal kings who were given (and corrupted by) the nine rings of power. The rings held the promise of near unlimited power and life but instead, over time, caused their mortal forms to fade until they had became wraithlike and completely under Sauron's command.
Interestingly, the Nazgûl have no physical forms and wear cloaks only to give themselves shape when dealing with the physical world, they themselves existing primarily in a ghostlike plane of reality slightly removed from our own.
Their sense of sight is severely diminished and they instead rely on their keen sense of smell and hearing. It was noted in the books that they may have some limited form of sight through the eyes of their mounts but this may be conjecture on the characters part. Those who wore the One Ring (an item the Nazgûl were tasked by Sauron with capturing at any cost) became plainly visible to the Nazgûl as the wearer shifted into their plane (and became invisible to others in the more physical plane just as the Nazgûl would be without their cloaks).
They rode coal-black horses until the incident near Rivendell when Elrond and Gandalf turned the river the pursuing Nazgûl rode into into a flood whose foam had the aspects of white horses and riders and washed them away. When the Nazgûl later reappeared riding huge winged mounts (provided by their lord and master Sauron) they were known as the Winged Nazgûl.
Also known as the Black Riders, The Nine, Nine Riders, Nine Servants, Winged Shadows,
and Úlairi.
Information obtained from: http://www.glyphweb.com/arda/