The direwolf is a relative of the normal wolf, though much larger and stronger. A grey direwolf on a white field is the sigil of House Stark. Direwolves were regarded as extinct and had not been seen on Westeros for nearly 200 years, until Robb Stark found a dead female direwolf with a litter of six pups, one for each of the Stark children.
The Stark children have formed a deep and powerful relationship with each of their Direwolves. Like other animals, direwolves can form a spiritual connection with skinchangers, otherwise known as wargs — people that are able to enter the minds of animals and control their actions.