1) There are no more tiles on the game board to select from. Every tile has already been selected, indicated by a check 窶懌㌃窶 mark. This is the way the game normally ends.
2) A player moves in such a way as to leave his opponent with no possible moves in that row or column. Since the opponent cannot make a move, the game is ended and the player with the most points wins. This is a good strategy to use when you are ahead in points and you want to end the game early and win. Of course you could do this if you were behind in points but you would then lose the game and that would be stupid!
If you are using Rotations and have some remaining, you will be allowed to rotate the board instead of ending the game when there are no moves in your current direction.
The player with the most points at the end of the game wins.