What a car alarm can do for you is to alert you, and anyone within hearing range, that your vehicle is being tampered with or someone is trying to break into it. Depending on the kind of alarm system you get, you may find that some alarms go off when someone approaches your car and gets to a certain distance from it. Other alarm systems go off only when the vehicle is touched or when someone attempts to open any of the doors, the trunk or the hood.
Some alarm systems use door sensors to set these off. This is not a pretty reliable car alarm to get since these only activate when the doors to your car are opened. Anyone can actually steal your car without opening the doors by simply towing it away, or even by breaking windows, if this is the kind of alarm you have installed. It would be better to get more advanced systems than this.
One advanced remote car alarm to consider is one that uses shock sensors on them. Any movement near or on the car sets off the alarm, so anybody who tries to strike, tow or even leans on your vehicle, the alarm will go off. Some alarms sound off differently for the different levels of movement the vehicle experiences. The more pronounced the movement, the louder and more pronounced the alarm.
Some people find that, aside from using alarms that are activated by shock sensors, adding pressure sensors to a vehicle is also a good theft deterrent. Pressure sensors are activated when a change in the air pressure of the vehicle changes suddenly, either with the opening of a door or the breaking of a window. Others even add motion and tilt sensors and perimeter scanners to their remote car alarm systems to ensure that other methods of stealing the vehicle are covered.