Most people, when they think of a bearing, would normally think of a ball bearing. In this type of bearing a fixed surface and a moving surface are separated by a series of lubricated balls that run in special tracks or races. Perhaps the most common application is when a shaft rotates inside a fixed hub, for example a drive shaft on a car or the front wheel of a bicycle.
In an air bearing, the balls are replaced by a cushion of air. Perhaps one of the most familiar applications of an air bearing is a hovercraft. Large fans blow air underneath the hovercraft and the air is prevented from escaping by a flexible rubber 'skirt'. The high air pressure generated under the hovercraft is capable of supporting its weight and it therefore floats on this cushion of air. The large cushion of air not only acts to support its weight but also as a soft spring which allows it to float smoothly over the rough surfaces on land or in the water.
It is possible to use this same principle to act as a bearing for a rotating shaft. High pressure air is fed into the gap between the rotating shaft and a stationary bearing. The gap is extremely small (around 1/100th of a millimetre) allowing the air pressure to be maintained within the gap. This small gap also significantly reduces the 'springiness' of the air cushion resulting in a shaft that is very accurately located i.e. has low dynamic run out. The shaft can then rotate freely as there is very little friction and the air pressure will ensure the shaft does not come in contact with the stationary bearing surfaces.