Airbrushes are usually classified by two characteristics. The first characteristic is the action performed by the user to trigger the paint flow while the second is the mechanism for feeding the paint into the airbrush.
Trigger: The simplest airbrushes work with a single-action mechanism where the depression of the trigger actuates air flow through the airbrush. Single-action airbrushes are simpler to use and generally less expensive, but they present limitations in applications in which the user wishes to do something more artistic than simply apply a good, uniform coat of color.
Dual-action or double-action airbrushes enable the simultaneous adjustment of both air and color at the trigger, by allowing the user to actuate air by depressing the trigger and simultaneously adjust color by sliding the trigger back and forth. This allows for greater spray control and enables a wider variety of artistic effects.
Feed system: Paint can be fed by gravity from a paint reservoir sitting atop the airbrush (called gravity feed) or siphoned from a reservoir mounted below (bottom feed) or on the side (side feed).