Alliances are a in-game system wich allows you to create and join an alliance and fight against other alliances.
When fighting, yuor goal is to take up the enemy's bases for earning war points. Visit Attacking Tips page to learn how to attack. then comes the defense. Follow those Tips and remember to fill the bunkers. Try to place your colonies far from your main planet. Always remember that if you attack from a colony it will be revealed to the enemies.