Even if you’re too young to remember the 1978 arcade classic, Space Invaders, there’s a good chance you’ve heard of it. You might have even played it at an arcade or an online gaming site. It featured an army of invading aliens marching in lockstep from left to right across the screen, making their way to your hapless spacecraft. The object of the game was to pick them off one at a time before they closed in.
source: gamezpot.com