Typically refers to a style of game where a player's ship can rotate 360 degrees on a 2d plane and is responsible for shooting and destroying on coming asteroids. The asteroids start out large, but divide into smaller obstacles as they are damaged. The game becomes more difficult and chaotic as asteroids are broken into the smaller pieces.
Shoot 'Em Up or shmup games are a subset of shooter games where the player continually moves forward while waves of enemies enter the screen. Both shooting and dodging are critical to advancing. See also Bullet Hell.
