To play the game, simply run the program. You will be placed in the game arena with a large block of aliens. To move your gun pod (at the bottom of the screen) use the left and right arrow keys. To shoot, press space. You can move faster if you hold the arrow keys down.
The object of the game is to shoot all the aliens (the multi-coloured moving things) before they reach the ground. Since these aliens aren't friendly (which is why you're trying to stop them) they will shoot back. Dodge the descending bullets or hang about under the large stationary blocks for cover.
The ninvaders program was written by Thomas Dettbarn <dettus@matrixx-bielefeld.de> and is released under the GPL.