You start each level with a flock of Sheep, and the aim is to "persuade" the Sheep to navigate their way to the level Exit while keeping as many of the little flockers alive as possible. Apart from wrestling with the sheer mind-boggling brainlessness of your charges, there are huge numbers of hazards to avoid and obstacles to overcome, and if you are sharp, quite a few juicy bonuses to be found.
You can choose to play one of four herders and you use the Herder to coax your Sheep into negotiating their way through the hazards.
You do not have direct control of the Sheep (not as easy as you thought huh?), but you are able to control the Herder. The key to success is to understand how the Sheep react to the Herder in all situations and so be able to use the Herder to drive the Sheep to where you want them to go.