An airbrush is a small, air-operated tool that sprays various media including ink and dye, but most often paint by a process of atomization.
Spray guns developed from the airbrush and are still considered a type of airbrush.

The first airbrush was invented in 1879 by Abner Peeler, in Iowa, USA.
An airbrush works by passing a stream of fast moving (compressed) air through a venturi, which creates a local reduction in air pressure (suction) that allows paint to be pulled up from an interconnected reservoir at normal atmospheric pressure.
The high velocity of the air atomizes the paint into very tiny droplets as it blows past a very fine paint-metering component.
The paint is carried onto paper or other surface.
The operator controls the amount of paint using a variable trigger which opens more or less a very fine tapered needle that is the control element of the paint-metering component.
An extremely fine degree of atomization is what allows an artist to create such smooth blending effects using the airbrush.

A well skilled airbrush artist can produce paintings of photographic realism or can simulate almost any painting medium.
Painting at this skill level involves supplementary tools, such as masks and friskets, and very careful planning.

Some airbrushes use pressures as low as 20 psi while others use pressures in the region of 30-35 psi.
Larger "spray guns" as used for automobile spray-painting need 100 psi or more to adequately atomise a thicker paint using less solvent.

They are capable of delivering a heavier coating more rapidly over a wide area.

However certain spray guns called, High-Volume Low-Pressure (HVLP) spray guns, are designed to deliver the same high volumes of paint without requiring such high pressures.
Source http://en.wikipedia.org/wiki/Airbrush