A Setter is the one in charge of the offense. She creates plays and directs the tempo of the attacks.
A Middle Blocker is the one responsible in the defensive end. She is positioned at the net to block the opponent's attack everywhere in the court. She can also be a Middle Hitter during the offense, usually on quick sets.
As the name implies, an Opposite is placed across from the Setter in rotation. She is accountable in helping to sets attack when the Setter is unavailable; and help out in blocking.
An Open Hitter is the main gunner of the team. She is the first option of the Setter during offense.
A Libero is the one responsible on floor defense. She can't serve nor spike. And usually wears a different color of uniform.