D.S.' are treated as regular players in terms of rotations and rules. They are subbed in when a front row player gets to the back row, then back out when the DS gets to the front row.
A libero is a special player with the same skills as a DS, but has a slightly different set of rules. Liberos can sub an unlimited amount of times, and in some leagues, cannot serve. (though this is uncommon) Liberos also cannot set a front row player in front or on the attack line, nor can they attack when the ball is over the top of the net.
In short words,
A DS costs the team a substitution and can only go in for one position (one person). A Libero can go in for any person so long as they are back row. Liberos can also serve now, but only in one position.
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A libero is a defensive specialist, meaning they only play back row. Typically, they sub in for a player who only plays front row (like a middle hitter) and they wear a seperate jersey, usually of a different color to distinguish them better.
A libero is a designated back row player for middle blockers because middle blockers are generally much taller and aren't as strong of passers. The libero can not play front row but they can go in for both middles. The best part about having a libero is you have unlimited substitutions so you can switch out any of your two middles back row as many times as you want without penalty, you cannot however have the libero go front row. Does that make sense?
A Defensive specialist has to be subbed in, but a libero can go in and out of the back row freely.