I found a great post here: http://home.earthlink.net/~tfakehany/system.html
Volleyball systems are referenced numerically. When describing a Volleyball system with numbers, the first number indicates the number of players who are primarily hitters and the second number indicates the number of players who are primarily setters, example: in a "4-2" offense, four of the players on the court are primarily spikers and the other two are primarily setters. The setter in the front row is the active setter, while the remaining setter plays defense and/or sets broken plays. There are three main systems used in Volleyball.
The first is the "4-2" as described above. This is very popular for lower level teams because it is simple, and the active setter usually doesn't have to worry about anything except setting the ball. Few teams above the high school level use the "4-2", although the USSR women's team ran a "4-2" in 1989 against the USA team, so anything is possible. The weakness in this system is that there are only two primary spikers in the front row and the opposition has three blockers, thus there is a major advantage for the defense.
What most teams want is a three hitter versus three blocker alignment. The "6-2" is able to provide this. We all know that the rules won't allow for eight players on the court. So properly, in the "6-2" system two of the hitters double as setters. In the "6-2" the active setter is in the back row. This allows the setter who is in the front row to concentrate on hitting. This is in contract to the "4-2" system where the active setter is n the front row.
When the ball is passed, the back row setter runs to the net to set (in a movement called a penetration) before returning to the back row to play defense. When the ball is passed, the setter in the back row will run to the net and set while the other setter is getting ready to hit. One setter is always in the front row and the other is always in the back row. They should never be in the front or back together.
Most teams use a "5-1" system which is a combination of the "6-2" and the "4-2" systems. With the complexity of today's game, most of the collegiate and international offenses teams have found it difficult to find two setters who are great at both setting and hitting. Furthermore, when you have two setters it can be difficult to switch between the rhythm of one setter and the other setter every three rotations. The Cuban National team and a few top college teams are the only teams who use the "6-2" at a high competitive level. A greater number of middle and lower competitive teams and most recreational teams use the "6-2" system. In the Eastern United States, a "6-2" is frequently called a "6-0". The minor difference between a "6-2" and a "6-0" is that the setter and opposite are substituted as they change rows. When the setter goes to the front row, instead of becoming a hitter, a new hitter substitutes in. This is a "6-2" because you are using eight players (6 2). In the "6-0" there is no substitution and you use six players (6 0).