The rules are well-known: a cell dies, if it has less than 2 or more than 3 neighbors; a new cell is born, if there are 3 neighbors in that place. You can edit a configuration by mouse clicks (press the ``Stop'' button to freeze the current configuration before editing). Press the left mouse button to add a new cell, right button to remove a cell, and middle button to toggle. The ``Start'' button initiates a life.
The source text of "Life" applet.