Home security systems work on the simple concept of securing entry points into a home with sensors that communicate with a control panel or command center installed in a convenient location somewhere in the home.
The sensors are typically placed in doors that lead to and from a house as well as easily accessible windows, particularly any that open, especially those at ground level. Open spaces inside of homes can be secured with motion sensors.
If your security system is professionally monitored by an alarm company, they are alerted when a security problem arises in your home. Along with the high-decibel alarm that sounds, the monitoring company is alerted. A trained security expert might attempt to communicate with the homeowner via the control panel if it's setup for 2-Way Voice communication, or will call the emergency contact number listed on the account.