Airbrushing gives you much more control over paint color, spread, and thickness as compared to spray paint. Many prop makers air brush so when replicating props finding the right color paint can be impossible with spray paint. However, airbrushing is more complex and expensive than spray paint.
An air brush consists of the body, needle, tip, air cap, air hose, and an air compressor. The air compressor pumps air through the body and when the trigger is pressed the air rushes out the air cap, down and into the tip, through the needle, and into the jar. The air escapes through a hole in the jar and in doing so pushes paint up through the air brush and out the needle and onto your project. The more narrow the needle and tip the finner the spray. Needles can be adjusted on their own to give a wider or finer spray.