NO (normally open) means that the internal switch is open when the magnetic module is against the switch module (door is closed).  NC switches are the opposite.. the internal switch is closed when the magnet is against the switch. Most security systems call for NC door switches so if the wire is cut the alarm system will detect the circuit opening and go into alarm state. Sensaphone monitors will accept either NO or NC switches but NC is advisable  so that the loop is supervised. NO switches are often used to activate a siren or chime when the door opens but magnetic door switches are designed to be used with alarm panels and will fail prematurely if used to directly power sirens. If you wish to have a siren connected to your door it is advisable to install a relay to carry the load..