Most sports train players have one position for which they hone their skills. People became famous for being quarterback, not for playing football. In volleyball, everyone is an all-around player. Although there are specialized players that are naturally better in one position than another, they play every spot on the court.
There are 6 people on a volleyball court at any given time; three in front and three in the back. The six play the game from those starting positions until a point is scored. The ball changes hands, and when it returns again to the first team they rotate, each player becoming the man (or woman) to his side (the back line moves to the left, the front line moves to the right).
The positions are rotated, but specialized jobs are not. They are as follows:
Setters. The job of the setter is not to get the ball over the net but to touch it into place for another player to drive the ball over. (Using the lingo of other spots, the setters make the assists.)
Left Side Hitters, Middle Hitters, and Right Side Hitters work as offense and defense. They are strong hitters that get the ball from the setter, but also play and important role in protecting their own court.
Liberos are THE defensive players. They can trade with anyone in the back row at any time the ball is not in play.
The formations in volleyball do change, but not in the way you'd expect. Players always stand in their 2 lines of 3; the variation is how many setters and hitters you have on the court. The 3 traditional variations are 4-2, 5-1, and 6-2.
4.2 variations have four hitters and 2 setters. The 5-1 formation has only one setter, which means that you will have 3 attackers in your front or back line at any time. A 6-2 formation is actually close to a 4-2, but the back row setter comes forward for each set. 6-2 is an offense-heavy set-up.