It depends on your positioning. Some guys will bend over at a 90 degree angle, in which case it's pretty much a lat excercise, hitting the forearms and bis as well. But to me, that angle is unnecessarily hard on your lower back. I like to be up a little higher. It's easier on your back and will involve the traps more.
But either way, bent rows, like any kind of row or chin, is primarily a lat excercise.