Originally Posted by Miyuki
I'm not sure either :x
Yandere: Someone who's extremely nice and loving on the outside, but goes into a psychotic rage every now and then. Often has jealousy issues, clingy, etc.
Tsundere: Someone who's spiky and irritable on the outside, but has a hidden soft side to them. This tends to be more common in anime, since so many of em have a girl that hates the main char, but eventually softens up to him.
Either way, I'd prolly prefer an yandere, since I already know plenty of tsunderes irl. :3