How does the libero work in volleyball?

Libero is a specialized defensive player.  They wear a contrasting jersey color from their teammates 

They only play in the back row. Very generally speaking their “job” is to take the first volley over the net and pass it to the setter for their team.  A libero can set a ball, but they cannot block and they cannot attack the ball when it’s entirely above the next (aka cannot spike).

Usually players need to notify an official about a substitution and a coach gets a set number of substitutions.  However, a libero can replace any back-row player without notifying an official and it doesn’t count as a substitution (aka an unlimited number of times).  But the only player who can replace the libero is the one whom the libero replaced.

Contrary to what the other says, the US official rules is that a libero can serve.  As soon as the libero serves in a certain rotation, they can only serve in that rotation for the rest of the game.

The libero wears a different color from the rest of the team and it is not allowed to serve