Stalemate occurs when a player's turn arrives, and he is unable to make a legal move. This situation is a tie! When you are ahead a whole lot of pieces against your opponent's lone King, make sure he always has a legal square to move to, so that you don't stalemate him. Then, when you have all your pieces in position, deal the final blow with checkmate.
Black moved his Rook to f8. Oops! Now it's stalemate and Black is forced to accept a draw! He should have played Qg2# or Ra1#.