Drag and drop a white piece to the board so it captures a black piece. Each move you capture any of the opponents pieces that are surrounded (vertically, horizontally or diagonally) by two of your pieces. The idea is to end up with more pieces than your opponent. If you run out of pieces, you can right-click to flip one of your opponent's pieces to flip it over.
Use the Style menu to switch between a two-dimensional or three-dimensional view.
Othello is a trade name for Reversi - if you like this game you can buy Othello or Excalibur Magnetic Travel Compact Design Ultimate Reversi from Amazon. For more information, there is a good wikipedia page.
If you have any trouble running the applet, read our instructions for installing java 3D.