Shapechangers are, apparently, magic practitioners that can alter their forms. It is unclear if they physically change their shape or are synonymous with skinchangers, those who can enter the minds of animals and control them. For instance, people who encounter a wolf controlled by a warg might mistake that the wolf's human master actually transformed into a wolf.
Jojen: Warg. Shapechanger. Beastling. That is what they will call you, if they should ever hear of your wolf dreams.
Bran: Who will call me?Jojen: Your own folk. In fear. Some will hate you if they know what you are. Some will even try to kill you.