How do air disc brakes work?

Most trucks in Europe have disc brakes front and rear now and some trailers do also. I run a Volvo wagon and drag outfit with disc brakes. Volvo, Mercedes, Scania, Daf (Paccar) and others have been using disc brakes on heavy trucks for 6 years or more now. They work in the same way as car systems do but using compressed air instead of fluid.