Shmup is short for, shoot - 'em - up. It is a subgenre of shooter video games, usually presented in top-down or side-scrolling perspectives.
Recent games like Death Smiles are considered shmups, along with recognizable games like Gradius, Raiden, R-Type, Radiant Silvergun, etc..
Here is the wiki :
http://en.wikipedia.org/wiki/Shoot_'em_up