Hitter: You play either on the left or right side in the front row. Some hitters may play from a spot in the back row. If this is you, with equal defensive and hitting skills, you may not be a libero but you could still be a back row player. You can select Back Row. Otherwise, your typical front row hitter is tall, athletic, can jump and hit well, and if your team blocks a lot you may be tasked with blocking. You could also be a middle hitter, the tallest on the team and usually the #1 blocker. You don't get set as much as the other hitters, though.
Back Row/Libero: In charge of receiving and digs. Must have excellent passing. Libero mainly specializes in passing, the others help out or may serve the ball (some schools and colleges allow the libero to serve in the US). May occasionally hit the ball from the back row, or can set the ball if it's too far for the setter (it's your job to make sure it's not in the first place!))
Setter: The leader of the team. One of the most challenging positions. Communicates a lot, is excellent at setting. The hitters can't hit the ball without someone who's good at setting it to them.