While security systems will vary depending on the need of the customer, they will all have the same basic principle of securing the entry points, like doors and windows (usually with sensors), plus motion detectors for interior detection. 

A central control panel works with the sensors and motion detectors and all other equipment that has been installed.  

A control panel - communicates with each installed component, sounds alarm if system is breached and contacts monitoring station

Digital keypad - these can be wired or wireless, you can control them from your smartphone, the choice is yours.  The keypad is where you arm and disarm your system.  It will also let you know if there is a problem with the system, such as window sensors open, battery issues, etc.

Door and window sensors - sends message to control panel if door or window is tampered with

Motion sensors - detect movement inside home and send message to control panel

A high-decibel siren or alarm (interior, exterior or both) - sounds when home/building has been breached to alert homeowner

Professionally Monitored Security Systems: If your security system is professionally monitored by our alarm company, they are alerted when a security problem arises in your home. Your high-decibel alarm will sound and the monitoring company is called by your control panel.  A trained security expert will call the emergency contact number listed on the account.

In the event of a break-in:  The monitoring station will call the first person on your contact list to verify that it was not a false alarm.  If they do not reach you, or if you verity it was not a false alarm, they will notify the appropriate emergency response personnel in your area. This includes police, firefighters, and paramedics. The monitoring company will also try to maintain communication with you until emergency response teams arrive at your home.

If it is a false alarm - lets say you forgot to disarm the security system before you let out Fido, you simply answer the phone and tell the person it was a false alarm.  They will ask your password and you tell them.  That's all.  NOTE:  Always be sure that everyone living in your home knows the password.  If it is not given to the security operator, police will be dispatched.  

Monitored systems typically allow for the homeowners (or designees) to be notified by text message and email when a security breach occurs.  Another advantage is being able to remotely arm and disarm your home from your smartphone.  

Remember......Non-monitored Security Systems: There are plenty of DIY security systems available today that don't include professionally monitored services. In the event of a home intrusion when this type of security system is installed, a high-decibel alarm sounds (provided one is installed). Police, fire and ambulance are not called because the system is not linked to a monitoring station.