There are several types of glass breakage detectors available: "Acoustic Detectors", "Shock Detectors", and "Piezo-Electric Vibration Detectors". Each of which have their specific advantages&disadvantages.
An older method which is no longer used, but may still be active on older systems is "Window Foil" - this is the small strip of lead or gold foil applied directly to the glass approximately 4" inside the window frame - this worked by literally tearing into&breaking the electrical connection when the glass was broken.
These are the most commonly used method for detecting glass breakage today. They detect glass breakage by literally listening for the sounds generated by breaking glass. Modern units are very stable and are not normally activated by extraneous sounds. Advantages: Very accurate when properly installed&adjusted, Low cost per unit and a single unit can detect an entire room full of windows, Small attractive units which can be placed on a wall or ceiling near the glass they are detecting / Disadvantages: Can occasionally be activated by sounds within the home or office they protect.no alarm will occur.