Dual action allows you to control the flow of paint. A single action shots the same amount of paint regardless of how far down you have the button pushed. Essentially with a single action its on or off and dual action is like having a dimmer switch. you can turn the lights (or paint in the case) down so low they are barely on or turn them all the way up to maximum brightness, if you catch my meaning.
Dual Airbrushes are generally preferred although some people will use a single action if they are just doing a large area with a single color.
Might want to watch this video. I think this was the one i watched awhile back that shed some light on a few aspects of airbrushing for me.
[ame="http://www.youtube.com/watch?v=B5nRNcNM16g"]How to airbrush for beginners - YouTube[/ame]