Rebirth allows characters to remort as their current race, gaining improved stat maxes (+3), as well as internal bonuses to their HP and mana and new leadership capabilities. They will be allowed to play a member of their same race. Otherwise, being reborn is the same as remorting as a new race; it can only be done once.
The leadership qualities benefit to rebirth results in being able to hold more members in your group, whether they be other players or charmed NPCs.
The rebirth quests are different for each continent. For Lensmoor, explore Kinson Manor. For Antrippa, explore Ashlish.