In electrical controls and safety devices, contacts can be either normally open (NO) or normally closed (NC). Normally open contacts are open circuits that close when the device is activated, allowing current to flow. Normally closed contacts, on the other hand, are closed circuits that open when the device is activated, interrupting current flow. Understanding the difference between NO and NC contacts is crucial when wiring electrical controls and safety devices, as incorrect wiring can lead to system malfunction or safety hazards.