Your spaceship is at the left side of the screen.
You can move it up and down with the up- and down-keys or with a joystick and fire with the ctrl key or the fire button.
The alien spaceships come from the right side.
If you shoot them up you will gain energy, but if they reach the left side you will lose energy.
Energy level is displayed as a bar at the bottom of the screen.
No energy = Game over