Most are little more then a glorified microphone that detects high pitch sound. Normally they are installed in the front area of a vehicle, either below the dashboard or above it.
One of the weak points of a glass break sensor is that they are not very effective in large vehicles, such as vans where a sensor in the front is unlikely to detect sound in the back, let alone protect against a sliding window being removed. They are also not recommended for vehicles where window tint film is fitted.
Window tint film muffles the sound of glass breaking. Glass break sensors can also cause a false alarm if the sensitivity is set to high. The materials inside a car can cause a creaking sound as they expand and contract with a change of temperature which can be enough to trigger a overly sensitive sensor.
Ultrasonic sensors are a far more effective and reliable way of protecting the internal space in a vehicle and would be my choice over a glass break sensor every time. Thatcham approved alarms (CAT 1 and CAT 2-1) require a movement sensor, such as ultrasonics or a microwave sensor, so glass break sensors are not approved under these standards.