"Air brakes" are brakes that increase a veichle's air resistance, making it less aerodynamic (though not much less stable). they work by increasing friction between a veichle and the air, slowing it down. These are normally found on aircraft, however, such as the Russian's SU27 (codename "Flanker") thus: http://www.abbotsfordairshow.com/history/timeline/su27-1991-3.jpg and http://www2.zzu.edu.cn/wzb/tpss/001/su27.jpg (Note the extender 'flap' directly behind the cockpit, central to the aircraft). "Air brakes" are not to be confused with "Pneumatic brakes", which are rarely seen in large road veichles, and which use compressed air or gas to push the brake bads onto the wheel.

This question deals with trucks and not aircraft, so the definition of an air brake would be a brake operated by a piston driven by compressed air.

Air brakes are simply the wheel brakes on the truck, but are powered by compressed air rather than hydraulics. I believe there are two types of air-brakes. Ones that use pressure to apply the brakes, and ones that use pressure to hold the brakes open. Normal car brakes operate by using pressure to apply the brakes, but if you somehow lose your brake fluid, you lose all braking. This is why some trucks have the opposite. The reasoning behind this is that if you blow an air line or lose pressure to the brakes, the brakes will apply and stop the truck. They won't come off again until they are pressurised. It's more of a safety factor... I don't like the idea of a truck with no brakes.

In the 1970's truckers were dying whenever they lost air pressure heading down steep mountain grades. The Department of Transportation and the Federal Bureau of Motor Vehicles Safety Division changed the requirements for air brake systems on all motor vehicles. It used to be that when applying brake pressure the air would push the brake piston inward to apply pressure on the brake shoes and wheel drums. If there was a loss of air, there would be no pressure available to apply. Truckers would have a runaway truck on their hands. In the late '70s or early '80s the engineering of these brake systems was reversed. Now when air brakes are applied, there is a valve that receives the applied air pressure. When the valve begins to open, it releases stored air pressure that is being held inside a chamber. That chamber has a heavy spring inside of it that begins to apply brake pressure in the absence of air pressure. Now when a trucker loses air pressure on a mountain grade, the spring will apply fully to lock up the brake shoes against the brake drum, preventing (hopefully) a runaway truck. Severe loss of air pressure does happen, but is not common.

AIR BRAKES ARE APLLIED BY USING AIR IN A CHAMBER MADE ONLY FOR BRAKE APPLING. THE BRAKE AIR PRESSURE IS COMPRESSED BY A COMPRESSOR IN A STORAGE TANK TILL NEEDED. WHEN THE BRAKE PEDAL IS PUSH FOR BRAKE APPLY, PRESSURE PUSHES THE BRAKES AGAINST THE BRAKE DRUMS TO STOP THE TRUCK. IF A HOSE BLOWS OUT, A SPECIFIC TANK IS MADE TO HOLD AIR WHEN THE OTHER TANKS ARE EMPTY. THE BRAKES NOW USE THE AIR IN THIS TANK TO ALLOW SPRINGS IN THE SYSTEM TO APPLY THE BRAKES. YOU HAVE APROX. 4 STOPS ON THE SPRING BRAKE TILL ALL THE AIR IN GONE. THE BRAKE ARE LOCKED TILL REPAIRS ARE MADE.