Airbrush is a painting method simulating traditional airbrush techniques. In Photoshop 6 and earlier versions, the Airbrush exists as an independent tool. In the later versions, it appears as Airbrush mode for various painting tools.
Note that the slower you move your tool, the more paint you put on the image. The paint flows even if you simply hold down your mouse key or press stylus tip without moving the tool (movie).
Tip. Just in case you didn't know, airbrush mode is also available in Photoshop Elementsthat that is way cheaper version.
And if you believe, Photoshop Elements lacks some other important feature, be sure to visit this page.