Air brushes generally either control only the air flow (single stage) or the air flow and media (paint, dye, ink, etc.). Single stage air brushes create low pressure in a nozzle which "sucks" the dye or paint into the air stream, spraying it onto the surface. You may change the density of the media by altering the air pressure, adjusting the nozzle or, replacing the nozzle and "needle" with a different size.
All these comments also apply to the two stage air brush however, it will also feature a more adjustments which my be varied while in use.
For a single stage I would recommend the Paasche or Badger. Two stage, definitely, the Iwata; expensive and worth every dime.