A legal move is to push a player token into a column from the top, or into a row from the left side. The move numbers are shown in the figure above.
A player wins by getting 5 in a line, either in a row, a column, or a diagonal. Thus, if the computer makes move number 3 in the above situation, the computer wins. We have a rule that if the moving player gets 5 in a line, that player wins. If the moving player doesn't win, but does make 5 in a line for the other player, then the other player wins.
When it's your turn, click a move checkbox. When it's the computer's turn, click away from the checkboxes (but in the applet). If you click in a checkbox and are running some versions of Netscape, the computer will move and then you will get the move in the checkbox, immediately, which makes the game harder to follow.
The program uses a lookahead method called minimax with alpha-beta pruning to make its moves. It is looking 4 moves ahead. It is possible for a human to beat it at level 4.