A draw may occur for a number of reasons, such as threefold repetition (the exact same position repeated three times), or the 50 move rule (a draw may occur if no capture has been made or no pawn moved in 50 moves).
A draw may also occur when one side wins on time, but has insufficient material to mate. A good example is here: http://en.lichess.org/UmBYdeys/black#112
the position is not stalemated as white's king has multiple moves available to him. White can possibly still win the position by promoting his pawn to a rook or queen and checkmating black. However, black with just a knight lacks the material to checkmate. Despite this, if black won on time, then a draw would be recorded.