How to Play Space Invaders:

Left arrow - Move left
Right arrow - Move right
Space - Fire laser
P - Pause/unpause game
Q - Quit game
S - Small scale version

You have to avoid the advancing aliens and shoot them with your laser. Besides trying to survive your other objective is to amass a high score. Mystery spaceships frequently fly from one side of the screen to the other. Destroy them and your score dramatically increases.