The game is played by moving a space ship horizontally at the bottom of the screen, while shooting the aliens that are descending to the bottom of the screen. Once all the aliens are destroyed a new wave of aliens begin their assault. The aliens attempt to destroy the space ship by firing at it while they approach. If they reach the bottom, the alien invasion is successful and the game ends.
There is more information on its home page.
You can download the source from github.