They are hard, but that is why we love them, right?
Typically, with an underhand grip, you pull the bar lower, say about to the belly. This works the lats more.
With an overhand grip, you pull the bar to your chest. This will hit the upper back more, including the traps and rear delts.
As far as angle goes, I am guessing you mean how far you bend over. Well, that is up to you and your spine. I feel, the closer you bend to 90', the more the target muscles are hit, and the less your lower back gets involved. Of course, this puts more strain on the lumbar area. Mix it up, go with what feels best.
I don't much care for bent over barbell rows. It never feels right for me. I prefer dumbells or a T-bar row. Once again, mix it up, each one targets a different area.