Its based on kills or damage inflicted in fighters and bombers.
Each plane has an ENY value if you look in the hangar screen. The higher the ENY value for the plane, the more points you will earn per kill.
If you fly a low ENY plane and shoot down a high ENY plane you won't earn many perks, BUT reverse that and you will earn lots.
I believe in fighters its the ENY of the plane you are in divided by the ENY of the plane you shot down.
You also earn perks for bombing stuff, running supplies and capturing fields(goons, m3)
SKurj