Systems may be self-monitored or central station monitored which means a live person in a monitoring center looks at each incoming video clip, assesses what has triggered the camera/detector, and then takes action if they see a human or vehicle when/where they should not be. Action could be calling people on the call list, calling the police/sheriff, or firing off sirens/strobes.
The problem with self-monitoring is you may not be available to respond to an incoming alert, meaning you miss the chance to catch the intruder. It's always better to stop crime before it happens!