This is the Vic-20 port of the arcade game. Galaxian was one of the era-defining "save-your-planet-from-invading-aliens" games. Unlike Space Invaders, the aliens swoop down at you and there are no barriers to hide behind. The game can get somewhat repetitive as there are no alternate screens and no "goal" other than a high-score to work towards. Still, for a stroll down memory lane, this is an enjoyable - if blocky - port of the game.
Atarisoft was a division of Atari that produced games for a variety non-Atari hardware.