A classic staple of The Golden Age of Video Games that has fallen out of favor in recent years. Everything is trying to kill you, and you have unlimited ammunition and a license to shoot first and ask questions later. Frequently, you're flying a spaceship or some other small craft, but other examples of the genre involve abstract shapes, people walking around with guns or bows and arrows, and so forth. The Ur Example is possibly Space War (although it's more of a one-on-one shooter rather than a shoot 'em up), the Trope Maker is Space Invaders, and the Trope Codifier is Galaga.