A sub-genre of video games in the side scrolling shooter genre, where the screen is usually COVERED in bullets.
These games usually consist of your player character at the bottom of the screen, dodging waves of bullets and shooting at enemies, until you reach the end of the stage where you are greeted by a difficult boss with even more dangerous bullet patterns.
These games are strangely fun in a masochistic kind of way.
Not to be confused with Bulletstorm.
Touhou is an example of a bullet hell game.