Will somebody please explain unit stats to me.

[Mandea]

yes and no, you'll inflict 80-100 damage to that stack, but not to 1 unit of that stack. if 1 unit has 70 life, with 80-100 you'll defenitely kill at least 1 unit out of x, maybe more if you score a critical hit.
if it has 200 life, you won't kill any unit.
come on, experiment a little, it's quite simple.