RE:@ tpugmire
Shooters should be both 1st and 3rd person shooting games [Gears of War, Call of Duty, Doom]. Also, Arcade style "Rail" shooters should be included in this genre [Time Crisis, Virtua Cop, Lethal Enforcers].
Shoot 'em ups, also known by shumps in gaming sites and forums, are formerly known as "side-scrolling shooters" and "top-down shooters" are the games that usually involve some kind of ship flying around and shooting at things on the screen. [Gradius, R-Type, Warning Forever, Super Stardust HD]. The debate is whether 3rd person flying games such as Star Fox be considered a Shoot Em' Up. It would most likely be the case as it is called a "Scrolling Shooter" in the US and Japan, and a "Rail Shooter" in Europe.
Unfortunately a lot of sites, and consoles no longer use the genre Shoot 'em up, and the classify those games under Arcade. Just one more reason why we should have that one sub-genre.