Lord Jon Snow born Aegon Targaryen is the son of Lady Lyanna Stark and Rhaegar Targaryen, the Prince of Dragonstone. From infancy, Jon is presented as the bastard son of Lord Eddard Stark, Lyanna's brother, and raised by Eddard alongside his lawful children at Winterfell but his true parentage is kept secret from everyone, including Jon himself. In order to escape his bastard status, Jon joins the Night's Watch and is eventually chosen as Lord Commander. However, his decision to allow thousands of Free Folk safe passage through the Wallalienates many of his black brothers, culminating in a mutinywhere Jon is killed but later revived by Melisandre. Freed from his Night's Watch vows, Jon executes the mutineers and joins his cousin, Sansa Stark, in building an army to retake Winterfell from House Bolton.
After securing help from a few other Northern Houses and the Vale of Arryn, they successfully retake the castle from Ramsay Bolton, restoring House Stark's dominion over the North with Jon being declared the new King in the North by the Northern Lords.
However, in exchange for the support of House Targaryen in the Great War, Jon surrendered the northern crown and was named Warden of the North.
(Sprice GOT wiki)