Well one player has 4 heroes out and tries to do missions while the second player is the villian and tries to stop them! On the next turn player 1 becomes the villian and player 2 is the hero. You win by scoring enough points as a hero or villian or completing 7 missions.
http://stargateonlinetcg.station.sony.com/freeplay.vmThat's the link to learn to play and play for free..