I couldnt find a description for it in the wiki ... what's the difference with essential?

A protected character can die in a fight. Followers are protected; in a fight, when their health becomes very low, they crowl on the floor and if you do nothing to help them and they get another blow they die.

Essential characters can't die.

I have checked that essential can take damages and when reach zero they get back to max ? Is it possible to set the npc to immortal status and no damage so untill it get disabled?

Protected actors will go down if they lost their last hit point because of a damage source not related to the player (if the player is the source they die), then if they recive more damage before they recover and stand up they die (other NPCs won't attack them, but a poison, a lasting damage spell or an accidental blow can kill them.

essential actors won't die, but they will prone down if their hit points are depleted, standing up again once they recover enough hit points.

Invulnerable actors won't get damage at all, but I'm not totally sure about damage originated from the player. You can use the setinvulnearble funcion (it operates on the base actor not in the actor itself)