The airbrush tool is a bit more complicated than the regular paintbrush. Instead of applying color to the image evenly, it applies it gradually. The "spray" is a semi-random distribution of pixels. Gradually, as the airbrush is left hovering over the same area while it is painting, it will fill up with color. A skillful artist can use this effect to create subtle differences in tone and variation, and even achieve the illusion that colors are mixing, all by using the airbrush. It does take some getting used to, however. The airbrush can paint at three different sizes, selected through the Options portion of the Toolbar.
Here's an example of some airbrush work. I used the airbrush to do the waves.