Side-scrolling shooter video games are usually action games that involve you and your gun progressing horizontally across a level, often in a very horizontal fashion. These games were extremely popular in the days of the Genesis/Mega Drive and Super Nintendo Entertainment System, before 3D became the standard and the mainstream decided 2D was outdated.
Side scrolling shooters are still made today, and not just on the Game Boy Advance or Nintendo DS. Although rare, developers still create side scrolling shooters for home consoles, such as Contra: Shattered Soldier.
Examples of Side-scrolling shooters