Those are not crows, but Ravens:
He [Sam] heard the dark red leaves of the weirwood rustling, whispering to one another in a tongue he did not know. The starlight itself seemed to stir, and all around them the trees groaned and creaked. Sam Tarly turned the color of curdled milk, and his eyes went wide as plates. Ravens! They were in the weirwood, hundreds of them, thousands, perched on the bone-white branches, peering between the leaves. He saw their beaks open as they screamed, saw them spread their black wings. Shrieking, flapping, they descended on the wights in angry clouds. They swarmed round Chett's face and pecked at his blue eyes, they covered the Sisterman like flies, they plucked gobbets from inside Hake's shattered head. There were so many that when Sam looked up, he could not see the moon.
Of course, the TV-show might have merged the two different occasions. In the books, Sam first faces the Other (the white walker) and kills it with his dragonglass dagger. He then faces the wight of Small Paul, where the dragonglass dagger fails him, but he is saved by fire and ravens and (possible SPOILER for season 4)
Coldhands, a wight in service of the Three-Eyed Crow. In the books, the Ravens are sent by the Three-Eyed Crow (who we know is a frequent subject of Bran's dreams in AGOT), who needs Sam to bring Bran through the Wall, because his servant cannot pass through the wall.
Are you sure you want to know the rest? If you do not know, I would wait for Season 4 and allow myself to be surprised by the (many) revelations that are bound to happen then.