An airbag system consists of the airbag module which contains an inflator, the airbag itself, and a trim cover. The proper firing of the airbag is controlled by crash sensors which supply electrical signals to a control unit which fires the inflator to inflate the airbag.
The driver-side airbag module is in the hub of the steering wheel. The passenger-side airbag module (if so equipped) is in the dashboard above the glovebox.
Ths is what happens when an airbag deploys in a crash:
In the 15 to 20 milliseconds after impact, the crash sensors and control unit determine the severity of the collision and decide whether to deploy the airbag.
At about 25 milliseconds, the airbag splits its covering pad in predetermined places and begins to inflate rapidly.
At about 45 milliseconds, the bag is fully inflated while the seatbelted occupant is still moving forward.
At around 60 milliseconds, the occupant contact the airbag which immediately begins to deflate via vent holes in the back.
Up to 100 milliseconds, the occupant continues to sink deeply into the airbag which cushions the head and chest while it is deflating.