Who is Jon Snow from A Storm of Swords and what is their importance?

Jon Snow from A Storm of Swords is the bastard son of Ned Stark. He is sent to join the Night's Watch at Catelyn's request after Ned is made Hand to the king. When Jon arrives at the Night's Watch he learns that his uncle BenJen has been missing for some time. He rises through the ranks to become a steward to the Night's Watch Commander. Later, when the commander is killed, he goes with Qyhoren halfhand to make contact with the wildlings. Qyhoren asks Jon to do what he must so that he can infiltrate the group, even if that means killing him. This is what exactly happens.

The wildlings accept him due to the result of the battle with Qyhoren and the fact that a wildling girl, Ygritte says that he's her lover. He is able to study them and to ascertain what is going on. When they attempt to scale the wall and attack the Night's Watch, he warns his brothers and he returns back to Castle Black, leaving behind the wildlings. He learns that the commander has been killed and there is a vote. Eventually, he is made lord commander.