The basic rule is as follows: The greater the wing slant (called dihedral), the greater the inherient stability of the aircraft in that axis.
This is hard to explain without diagrams. Imagine a wing that has no dihedral. As the aircraft banks, both wings are producing the same amount of horizontal and verticle lift. Now imagine a wing that has considerable dihedral. As the aircraft banks, the inside (low) wing is more level than the outside (high) wing. Since the inside wing is more level, it produces more verticle lift than the outside wing. Because of this inequality in lift, the inside wing goes back up and the wings become pretty much level again.