Singleplayer is where you play against AI tribes. There are currently two different singleplayer modes: Perfection and Domination. Each game mode has 4 difficulties: Easy, Normal, Hard, and Crazy. In singleplayer, the enemy tribes are randomly chosen from the tribes you have.
In domination mode, your objective is to defeat all other tribes in as little turns as possible. Your rating in domination is calculated based on the turns you took, the units you lost, the tribes you defeated, and the difficulty.