A two–way car alarm system is a system that not only delivers a message to you that someone is attempting to take your car, but it also allows you to do something about it. For example, some two-way systems allow you to turn off the ignition system with the touch of a button, while others let you cut off the amount of gasoline available for a quick getaway. Also, if the system set up right, with the correct GPS sensors, you will know where your car is so you can inform the police of its location.
For more info, see What Is a Silent Car Alarm