The libero, mentioned in other pages, is usually the best on the team at passing which is why they are the strongest defensive players usually.
A "dig" involves a player diving (sliding) on the floor and somehow getting under the ball and not letting it hit the ground. Digging usually occurs when on the defense and the offense spikes a ball at you and it is very tricky to get the ball.
Here is a photo taken off flickr.com of a player in action passing the ball