The wildlings remain a mysterious bunch despite being mentioned in two novels and hundreds of pages. But Jon Snow finally gets properly introduced to a few of them in A Clash of Kings. They are:
Ygritte is a wildling scout Jon Snow captures. Qhorin orders Jon to do what needs to be done, and once his brothers leave them, Jon releases the girl—when asked why, Jon says there was no evil in her despite her being his enemy (54.Jon.22). She is part of Rattleshirt's hunting party later in the novel, and she vouches for Jon Snow when he turns to the wildling side.
Craster is a wildling who will occasionally help the Night's Watch for a price. He lives at Craster's Keep, which is less keep and more giant hall made of sticks, mud, and cow pies (i.e. daub-and-wattle). When Mormont visits him, Craster informs the Lord Commander that Mance is gathering his forces at the Frostfangs.
Craster styles himself a free man who serves no one but Craster (24.Jon.50). Additionally, he marries all of his daughters and sacrifices his sons to the cold gods. He currently has nineteen wives. Er, daughters. Er… ugh.
Gilly is one of Craster's daughter-wives, and a pregnant one to boot. She befriends Sam and asks Jon Snow if he'll help her escape—she fears her child might be a boy, and if it is, Craster will sacrifice it to the "cold gods" (24.Jon.140-42). Gives us the shivers just thinking about it.
Mance Rayder is an ex-brother of the Night's Watch and the King-Beyond-the-Wall. We've been hearing a lot about him these last two novels but have yet to meet him. We do get some new information, though: Qhorin reveals that he used to be Mance's friend but that Mance abandoned his post because he "was not a man whose knees bent easily" (54.Jon.15), and was truly a man of the wild.
This wildling gets his name because his armor is made of bones. He doesn't trust Jon Snow's turn of heart to join the wildlings' cause (69.Jon.111).