In term of gameplay it really simple, all you have to do is aim (or click on target if you're on PC) toward the enemies until they die. Most of the stages are simple just as kill anything that moving, and sometimes there are other condition to win like don't let enemies pass a certain point, etc. For the boss fight there usually mechanics that you need to do to avoid attacks that do really high damage.
The main attraction of this game are the Stories (it really is interesting setting at least for me), Music's (they are really good), and Characters (you know what I'm talking about).