The shmup, or the shoot'em up, is the oldest well-defined game genre. Dating all the way back to Space Invaders, Tempest or River Raid, the principle is simple: you're controlling a spaceship (or a plane, or a magical girl) and shooting at things that come your direction while dodging whatever they fire in your general direction. Do games get simpler than that?
I have a special place in my heart for shmups because of all the time I've spent with them as a kid, starting with the legendary Raptor. The shmup, together with the platformer, helped define my concept of entertainment. It also taught me the imporant truth that games can be hard... and that it's okay if they are. (Starting over is fun.)