Space Invaders for the Atari 2600 was the first ever arcade to home game license, becoming a huge best seller.
Even though the basics of the original arcade game remained intact -- invaders would march from one side of the screen to the other, dropping down a notch at each edge, they would fire at the player, there were bunkers for the player's laser cannon to hide under, etc. -- there were quite a few differences with the basic setup, as well as with its many game variations.
This time around, this version had 36 onscreen invaders rather than 55 like on the arcade original. It also had six different kinds of invaders, rather than three, plus they are also worth different point values. There are also only three bunkers on this version (rather than four), but once the invaders started getting close to the Earth (another difference is that this version took place on Earth, rather than on the moon), instead of the invaders erasing them bit by bit as they plowed through the bunkers, the bunkers would just disappear altogether at once.