The object of the game is to own more pieces than your opponent when the game is over. The game is over when neither player has a move. Usually, this means the board is full.
Playing the game
The game is started in the position shown below on a reversi board consisting of 64 squares in an 8x8 grid.
Each reversi piece has a black side and a white side. On your turn, you place one piece on the board with your color facing up. You must place the piece so that an opponent's piece, or a row of opponent's pieces, is flanked by your pieces. All of the opponent's pieces between your pieces are then turned over to become your color.
A move consists of placing one piece on an empty square. If you cannot make a legal move (i.e. you cannot capture any of your opponent's pieces), you have to pass, and your opponent is on. If none of the players can place a piece, it means the game is over, and the player owning more pieces wins.
You can capture vertical, horizontal, and diagonal rows of pieces. Also, you can capture more than one row at once.