Your Guide to Westeros

Wargs

-this part is taken directly from the Game of Thrones Wiki to save time-

Wargs are people with the ability to enter the minds of animals and perceive the world through their senses and even control their actions. The act of doing this is called "warging".

The ability starts out as vivid dreams in which the warg sees and perceives the world through the senses of an animal. Over time and with practice, a warg can enter the mind of an animal at will. A warg's human body will become comatose when his consciousness leaves it to enter the mind of an animal, and his eyes will glaze over white, until his consciousness returns to his own body.

Warging is a separate ability from Greensight, the psychic ability to perceive future and past events in dreams. However, some people who are wargs can also possess greensight.

Wargs cannot easily enter the minds of other humans to control their actions, though few even attempt it. Bran Stark, with no prior experience at warging, was able to enter the mind of another human, demonstrating his powerful innate abilities. The human whose mind he entered, however, was Hodor, a mentally challenged man, and as yet no warg has been seen totally taking over the mind of another person who possesses full adult mental capacities.

Second Life
It is incredibly traumatic for a warg if the animal whose mind they have entered dies while they are controlling it, but the warg will survive this. If a warg's own body is killed while entering the mind of an animal, however, the warg's human consciousness can live on inside of the animal. When Jon Snow killed the wildling Orell by running him through with his sword Longclaw, with his dying breath Orell managed to warg his mind into his eagle. While Orell's human body died, his consciousness survived in the eagle, and proceeded to claw at Jon's face with his new body's talons until Jon was able to fend him off.

Known Wargs
Orell, a wildling from Mance Rayders' army. His human body was killed by Jon Snow. Now inhabiting his eagle.
Bran Stark, also possesses the power of greensight.
An unidentified {Thenn Warg}, a member of Styr's warband. Shot in the head with a crossbow by Samwell Tarly during the Battle of Castle Black.
Varamyr