Calvinists believe in pre-determinism, which is that it is determined at birth whether a person will be worthy of being saved or not, and their life will simply follow that destiny. This is unusual for Christianity in that most Christian churches believe in the idea of free will, that each person can choose whether to do the right thing or not, and will be saved or condemned depending upon the choices made.
I don't know if this is still applicable but Calvin himself preached a doctrine of double pre-destination, not merely single pre-destination. Double pre-destination is where God actively has decided who goes to heaven and then who goes to hell rather than merely deciding those he saves and letting the others fall to the consequences.
