In a 5-1 offense, a team has 1 setter and 5 hitters. As a setter you are responsible for setting from every rotation no matter where in the rotation you are - right back, left front, wherever.
The 4-2 offensive system features four attackers and two setters. It's the most basic offensive system. The two setters play opposite one another on the court. The setter that is on the front row sets for that rotation. With a 4-2, it's much easier to adjust the formation because the setter lines up near the target because they are always a front row player.
A team may play a 6-2 formation when you have 2 good setters and every player (including the setters) can hit the ball. In the 6-2 system, you always have 3 attackers in the front row, and your primary setter is always coming from the back row. The reason why it's called 6-2, is that you have 6 attackers, and 2 setters.
[This message has been edited by S.S.Aggie (edited 9/7/2008 6:13p).]