Shooting Games (STG)
Shooting games, also known as "shoot-em-ups", "shmups" or "STGs (ShooTing Games)", are a genre of video game that focuses primarily on a single player shooting down waves of enemies in a typically auto-scrolling fashion. Shooting games have two primary subgenres of games: Horizontal, also known as Yoko (Gradius, R-Type, etc.), & Vertical, also known as tate (DonPachi, Ikaruga, etc.), as well as the occasional Isometric shooter (Viewpoint).
The STG genre primarily originated in the arcades, and were often well known for their high level of difficulty (often ramping up greatly by stage 3), blistering-fast bullet speeds, and highly detailed artwork. As time went on, developers such as CAVE began innovating on the previously-established STG formula by introducing greater amounts of bullets, complex bullet patterns, smaller hitboxes, and complex scoring systems -- these games would be known as the "manic shooter" or "bullet hell" subgenre of shooting games.