Definitions
Region: any distinct area of the board, consisting only of empty points, and surrounded by one color of stone in conjunction with at most 2 board edges.
Loose stone: any stones not outlining a region. Loose stones are captured when surrounded by an enemy region
Objective: Control the most territory (stones + regions). The game ends with two successive passes
Gameplay: Players alternate placing a stone of either color on an empty point of the board. If doing so forms any regions the following conditions must be met:
all enemy stones within the prospective regions are loose stones.
there must be at least one loose stone in each prospective region to be captured.
Note: You may never play stones into an established region
Pie Rule: On his first move, the second player may opt to play as the opposite color in lieu of making a move.
—description from the designer