The object is to save your little flock of 10 sheep from the marauding Aliens who are dropped off at various sites and work their way toward the sheep. You save the sheep from abduction (and possible molestation - hence my inference that these particular Aliens are from NZ) by strategically placing different weapons around the map to intercept and hopefully destroy the Aliens.
It sounds easy enough, but there are different types of Aliens that require different strategies and weapons for success which keeps it challenging. Henry has watched me play a few times now but I've had to stop playing in front of him because he keeps yelling out "Baa Baa!" and trying to touch the sheep or just steal the iPad.