Several of the main point of view characters of the series are members of House Stark. House Stark is the principal noble house in the North; many lesser houses are sworn to them. Their seat is Winterfell. Their sigil is a grey direwolf on a field of white, and their house motto, referred to in-universe as their words, are "Winter Is Coming."
The Starks are descended from Brandon the Builder, a legendary figure from the Age of Heroes thousands of years in the past, who built Winterfell and the Wall. They were Kings of Winter in the North from the Age of Heroes beginning with Bran the Builder until Torrhen Stark submitted to Aegon the Conqueror at the end of the War of Conquest. Since that time the Starks have held the North for the kings of Westeros as Lords of Winterfell and Wardens of the North.
After Robert's Rebellion, the Starks have become a relatively small Great House. They share a common ancestor with the Karstarks, and may have distant relatives elsewhere in the North, possibly in White Harbor and Barrowtown.
ALLIED HOUSES: Baratheon, Tully
ENEMY HOUSES: Greyjoy, Lannister, Targaryen