Those are liberos, which are defensive players. I'm pretty sure they can't strike the ball overhand (outside of serves), so they primarily dig and set.
Incorrect. They are defensive specialists, but rarely do the setting and can, if they want to strike overhand. The issue is libero's are almost always much shorter than the girls you want todo the hitting.
The person on the team who usually does the setting is called the Setter. There are exceptions to this, depending on where the balls is relative to the setter, etc. but the most typical scenario has the libero "bumping" the first shot over-the-net to the setter, who then sets for the hitters in the front. That's why you'll see the Setter playing all over the court. She goes toward the ball so she can place it just right for her strikers.
Bumps are when you see the player hit the ball underhanded, with hands clasped together in an effort to hit the ball with both arms, rather than fists. Often they have to dive when the ball is too far away to "bump " it, so it could be just one hand or fist.