Manipulate the evolving cells to advance your followers and destroy the enemy. Here's a screenshot of the game. The above board is quite small - you can go up to 60 x 60, and the rules can be customised in quite a few ways. New cells are born when three live cells touch an empty square. The colour of the new cells depends on the colour of its three 'parents', but this does not affect the pattern of cells born.
Some patterns are stable, some expand strongly, and some die off over time.
A new cell is green or red depending on whether more of its parents were that colour - otherwise it is neutral. Soon all the neutral cells are gone, and Green and Red slug it out, trying to expand patterns of their own colour into empty territory, and killing off the enemy patterns. For your turn, you place a single cell of your colour in an empty square - then the board evolves one step. Now your opponent takes his turn, and so on. In the standard game, you must play beside a living cell, and you can't play in any cell touching an enemy cell unless it also touches one of yours.