The single action airbrush controls airflow with the trigger to atomize the paint mixture, while the needle controls the paint mixture volume. In a double action airbrush, the trigger controls both the paint volume and the airflow.
As its name describes, the internally mixed airbrush mixes the paint within the body of the airbrush, while the externally mixed one pulls paint from a cup or container. Internally mixed airbrushes provide finer detail and finish, better suited to models.
The feeding of the paint to the gravity-fed airbrush occurs via a cup attached to the side or top of the airbrush. A bottom or siphon feed works on the principal of suction or siphon, with a cup or bottle attached to the bottom of the airbrush.
The relationship of the needle to the nozzle is what controls the fine or thick lines of an airbrush. These are usually interchangeable to different sizes depending upon the art project.