Wargs, also referred to as "skinchangers", are people who are able to enter the minds of animals and control their actions.
Being a warg makes way for changes in the personality of the human and the animal involved. In addition, a warg bond is easier if the human already knows and is close with the animal, like Bran Stark and Summer. Usually wargs begin by entering the mind of the animal in their dreams. If an animal is killed while a warg is inside its mind, it is traumatic for the warg, and if a warg is killed while his or her conscience is inside the animal, some of his or her conscience may remain in the animal.
Bran Stark is the most skilled warg in the Stark family, and has repetitive dreams in which he enters the body of his direwolf, Summer. This began mostly after he became unconscious and crippled. Jojen Reed, a visitor from House Reed, helped him learn about his abilities. Kerowyn Milla, a rarely mentioned character, also learned from this visitor.