According to some, Airbags are the most important safety device since the seatbelt. Over the last ten years, the feature has been introduced into cars in the UK and has reduced the number of serious accidents on the road dramatically. Other 'System Restraint Systems' (SRS) found in modern cars are Seat-belt Pre-Tensioners, Side Impact Airbags inside rear door pillars and even airbags in the doors themselves.
In short, controlled explosives. As a vehicle has an accident over a certain speed (20 mph) a sensor will trigger the Airbags and other systems immediately. The typical airbag is situated in the middle of the steering wheel along with the inflator. When the on-board sensors have received information of a deceleration of around 5 mph and any impact to front, sides or rear of the car, a compressed gas capsule is ignited and the inflation process of the airbags takes less than one second to be completely deployed and restrain the occupant.