An overhand is a looping power punch that is generally thrown to get around an opponent's guard. When you throw it, you pull your right hand back a little bit and up a little bit. Then you throw an arcing punch that's similar to an overhead ax chop, but instead of aiming at a log on the ground, you aim at your opponent's head. One example of this would be the finishing punches Larry Holmes used against Gerry Cooney. Larry wasn't known for overhands, but Cooney was dead tired and not coming out of his guard to fight. So he just bombed shots around his guard to finish him.
The right hook is not commonly used by orthodox fighters as its range is so limited that it's only use is for in-fighting. When Hagler boxed out of the orthodox stance, he'd use the right hook while in-fighting. A hook travels a different arc as it comes from the side and travels in a hooking (hence the name) direction. You'll see sloppy bar-fighters throw right hooks from range. The punches are easy to see coming and gives the opponent a lot of time to react. A good fighter will demolish a careless right hooker.
