Critical Mass (aka Critter) is a shoot'em up (smup) similar to Galaxian. The basic idea is simple: you move your ship back and forth at the bottom of the screen, shooting up, while bad guys fly around and shoot at you.
Your world has been infested by an aggressive army of space critters. Overrun and unprepared, your government was unable to defend its precious resources. As a last effort to recapture some of the 'goodies', you have been placed into a tiny spacecraft and sent after them.