Acording to Wikipedia
Shoot ‘em up is a subgenre of video games within the shooter subgenre in the action genre. There is no consensus as to which design elements compose a shoot ’em up. Some restrict the definition to games featuring spacecraft and certain types of character movement; others allow a broader definition including characters on foot and a variety of perspectives.Basically, if your ship (or character) is flying and must blast everything, then it's a Shoot 'em up.
So yeah, discuss about your favorite shmups here.