it's generally a draw, but I believe there are five winning positions, all with the king on the edge of the board. The drawing idea is to use your rook to pin his knight against the king.
I played one recently too. I went into it knowing it was a draw. http://www.chess.com/echess/game.html?id=9630778
Here's a fun game http://www.chessgames.com/perl/chessgame?gid=1070866
I recently (happily) got a draw with KR vs KRN. I don't know the theory behind this. Is it drawn with correct play? Can KRN win with even average class player play or is it a waste of time?