Humanoid shape-shifting entities, acting as the succesors of previous iterations like themselves (The T-1000 being a more advanced model of previous Infiltrators models, such as the T-800 and the T-900, and Twelve being the succesor to Eleven), developed for violent purposes by a powerful "group" (or whatever you'd call Skynet) working behind the scenes (The Secret Society creating Twelve as a weapon and Skynet creating the T-1000 as an assasin). Said "groups" also sent them on a missiom to track down another character (John Connor and Necro), to which they follow without question. Due to their shape-shifting abilities, they can take on the form of other people and can even make weapons out of their bodies.