The end of the game
A game is lost for the person that is to move when he has no valid move to play. This often happens when a player has no pieces left, but it can also happen when the player has many pieces left. In the diagram below the position is lost for black.
When neither of the players can win the game is a draw. The official rules of the game formalize the situations in which a game is a draw.
Official rules KNDB