Cruel and Unusual Punishment

The eighth amendment states that, "Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted." But what exactly constitutes cruel and unusual punishment? Is capital punishment a form of cruel and unusual punishment? Are certain methods of killing criminals cruel and unusual (e.g., by firing squad, quartering, etc.)? What distinguishes the punishments that are cruel and unusual from those that are not?