The direwolf is the symbol of House Stark. So when Eddard finds a litter of pups on the road to Winterfell, his children convince him to allow them to keep the animals who happen to number four males and two females same as the Stark children.
Over the years, we've seen these direwolves grow loyal to the Starks—Ghost to Jon Snow, Nymeria to Arya, Lady to Sansa, Grey Wind to Robb, Shaggydog to Rickon, and Summer to Bran.
An adult direwolf may grow as large as a small horse and can tear a man to shreds. As we enter the eighth season, only Ghost remains alive at Winterfell though Nymeria may still roam the Riverlands.