Collect refugees who wave at you from the ground, fly over the runway in front of the tower where they'll drop off and run home.
Look out for the abductor UFO's which try to kidnap the refugees and transform them into cyborgs who will relentlessly track you down and harass you with photon bullets.
If you can catch a refugee dropped from an exploding abductor before he hits the ground you'll get some bonus points just for having style.

Move with the arrow or ASWD keys, fire with Z or M keys, pause with SPACE.
You can also pause by just moving the mouse outside of the game window.