The Savior is a third-party role whose objective is to prevent a specific player from being voted out. These protected players are called Savior targets; the fact that they are Savior targets is usually hidden from them.
If the Savior dies before the Savior target, the Savior can still win the game, but their fate is now out of their own hands.
If the Savior target is eliminated, the Savior loses and leaves the game.
The Savior target is almost always Town - otherwise, the Savior almost necessarily wins with scum.
There is no consensus as to what happens if the Savior target is nightkilled.
Sometimes a Savior will have more than one Savior target assigned to them. All of them must be protected. An Savior with this kind of Win Condition will usually have some kind of protective ability to help stop night-kills that may fall on their Savior targets.
Alternatively, a Savior will be charged with making sure at least one of a group of Savior targets survives to the end of the game.
Use and Power
This role has been criticized as being luck-based - drawing a VI player for a Savior target is horrible, while getting a particularly well-known player for a Savior target means the Savior doesn't need to do anything.
Aside from being a setup-based protection on a certain player, it serves no purpose except a challenge.