Burgler alarm is switched on, if bad person enters property, burglar alarm make loud noise, scares bad person away (or men with flashing lights on their cars turn up and take bad person away (Yeah right!))
Chuck in Essex men might turn up with flashing lights on their cars and take bad men away. In the rest of the country they only come if you threaten to shoot said bad men.
Quite simple guscbe25. Alarm systems work on contacts and a continuous circuit (for those with door alarm sensors). If the circuit is broken, alarm goes woo woo woo. For those with infra red sensors, there is an infra red beam, which if broken, alarm goes woo woo woo.
Not to mention those sensors which detect a strong vibration e.g. someone hitting a door or window to gain entry. This will avctivate the alarm, hopefully before access is gained. To my mind, a better scenario than allowing damage to be done after the intrusion. Though it has to be said a loud thunderclap will do the trick!
There also floor pads; heat sensors; motion sensors and many others, depending on how secure you want the property to be.
Nowhere can be absolutely guaranteed secure, as a brief perusal of the newspapers will show.
All burglar (intruder) alarms generally work on having a stable but fragile electrical circuit which will activate an alarm when that circuit is disturbed beyond a specified limit.