The White Walkers are a mythological race mentioned in ancient legends and stories from the time of the First Men and the Children of the Forest. Eight thousand years before Robert's Rebellion, a winter known as the Long Night lasted a generation. In the darkness and cold of the Long Night, the White Walkers descended upon Westeros from the farthest north, the polar regions of the Lands of Always Winter. None knew why they came, but they killed all in their path, reanimating the dead as wights to kill the living at their command. Eventually the peoples of Westeros rallied and in a conflict known as the War for the Dawn, they managed to defeat the White Walkers and drive them back into the uttermost north, with the Wall raised to bar their return.
In the present day, most believe they never existed and are just myths, spoken of in the same breath as ghosts, goblins, grumpkins, or snarks. Even the few who believe they did once exist think they went extinct thousands of years ago. Certainly, none were seen for thousands of years after the Long Night.
Just before the outbreak of the War of the Five Kings, however, disturbing reports began to reach the Night's Watch from the wildlings who live beyond the Wall that the White Walkers had returned. Having enjoyed a decade-long summer, Westeros seemed due for an equally long winter, and the White Walkers seem to be spreading with it. With the Seven Kingdoms embroiled in a petty civil war, the dwindling and under-supported Night's Watch realize that they are all that guards the realms of men against the return of these legendary demons of ice and death.
White Walkers possess the magical powers related to ice and cold. Their arrival is usually accompanied by blizzards and the dropping of temperatures. They can also freeze anything they touch, as one froze Sam's sword to the point that it shattered. White Walkers also have superhuman strength, as one managed to toss Samwell Tarly several feet away with a single back-handed punch. The White Walkers wield swords and spears made from unique ice crystals.
However, one of their most deadly abilities is to reanimate the dead as their servants, known as Wights. They are actually capable of reviving any dead animals as wights, as a few White Walkers have been seen riding undead horses. They cannot, however, revive a corpse into servitude if the fallen creatures have died by fire. Once the Wights have been risen to serve the White Walkers, their eyes turn an icy blue, similar to the White Walkers' own eyes. Wights can only be killed by fire and serve the Walkers without question.
As previously mentioned, one White Walker, speculated to be their master, possesses the ability to change humans into White Walkers, demonstrated at the end of Oathkeeper when he takes Craster's son into his arms and places a single finger upon its cheek, causing the baby's eyes glow blue and his skin to grow pale, taking on the appearance of the White Walkers. Whether or not this ability extends to only this White Walker, all of the White Walkers amongst his caste, or all White Walkers in general remains to be seen.
During the massacre at Hardhome, White Walkers are shown to be resistant to fire due to the extreme cold they radiate, which snuffs out any flame they approach. It is unknown if this also applies to wildfire or a dragon's fire.
According to legend, the White Walkers speak a language known in myth as "Skroth", which sounds like the cracking of ice.
At the start of the series, there have been peculiar rumors from beyond the Wall mentioning the White Walkers. According to Will, a renegade from the Night's Watch taken captive near Winterfell, his patrol was ambushed by several White Walkers and his comrades Gared and Ser Waymar Royce were killed by them. Prior to this incident, the White Walkers had massacred a tribe of wildlings and left the corpses out as a warning. His story was not believed by Lord Eddard Stark, who executed him as a deserter. The White Walkers showed an ability to animate the corpse of a dead wildling child as a wight.
Lord Commander Jeor Mormont and Maester Aemon of the Watch ask Tyrion Lannister to persuade his sister to send them reinforcements. They claim there is something other than the wildlings beyond the Wall and it's been growing in influence for some time. With winter coming they fear this unseen enemy will make its move
A band of wildlings fleeing south of the Wall passes close to Winterfell. One of them suggests taking a Stark captive for Mance Rayder, the King Beyond the Wall, but the others refuse to return north due to the dangers posed by the White Walkers, instead asserting they should head as far south as possible.
Osha, one of the wildlings taken prisoner at Winterfell, reveals to the Stark household that she and her companions were fleeing South to escape the Walkers. She also reveals the Walkers are nocturnal and sleep in the day and hunt at night. When her words are dismissed by Maester Luwin due to the belief the Walkers are long dead, she reveals that they were only sleeping and "they're not sleeping now".
The bodies of several rangers are discovered a short distance north of the wall and are identified as Night's Watchmen who have been missing for weeks. Despite this Sam notes that the bodies show no signs of rot and appear in perfect condition. Later that evening one of the bodies reanimates as a wight and attempts to attack the masters. The creature is stopped when Jon discovers the creature's weakness is fire.
After disposing of the remaining bodies, Sam tells Jon that he read that only the touch of a White Walker could do what they have seen tonight. A worried Sam notes the weakness of the Night's Watch and if the White Walkers decide to come in force then they are in real trouble
After recent events Lord Commander Mormont decides to mobilize the Night's Watch to head out beyond the wall to assess the situation and if necessary make a preemptive strike against their enemies. He explains to Jon that the war in the South is of little concern in relation to the threat of the returning White Walkers.
Jon Snow follows Craster into the woods and sees him placing a newborn baby, one of his sons, into the ground and leaves. Moments later a figure with glowing blue eyes appears and takes the baby away.
After Craster discovers Jon, attacks him and orders the Night's Watch to leave his roof, Lord Commander Jeor Mormont confronts Jon, though he also admits he's aware that Craster worships the White Walkers and sacrifices his newborn sons to them in exchange for safety for him and his daughter-wives.
Samwell Tarly, Grenn, and Eddison Tollett gather dung around the Fist of the First Men when they hear a horn being blown and believe Jon Snow and Qhorin Halfhand have returned. Then they hear a second blow, the signal for incoming wildlings, and draw their swords for battle. Then they hear a third blow, the signal for White Walkers. The three run to the Night's Watch camp but Samwell is left behind, while a blizzard envelops the entire plateau. He seeks refuge behind a rock while a White Walker leads a vast horde of Wights. The White Walker finds Sam but for reasons unknown, ignores him, riding past him atop an undead horse and leading the horde for an invasion.
A white walker attacks Samwell Tarly and Gilly when they are seeking refuge in a destroyed cabin. Samwell tries to stop the white walker with his sword, but the walker shatters it and throws him aside. The walker then approaches Gilly, as if going for the baby that she is holding. Samwell, on the ground, finds one of the dragonglass daggers that he found at the Fist of the First Men. He attacks the walker from behind with it. The walker then falls to the ground and seems to freeze after which he shatters in a cloud of icy dust.
Rast sets the last son of Craster on the ground in the Haunted Forest. A White Walker, riding an undead horse, takes the baby to a mysterious place in the far north, and sets him on an altar made of ice. Another White Walker with pointed horns on its head approaches, picks up the baby, and touches him on the face below his eye. The baby's eyes turn bright blue, similar to that of the White Walkers.
The White Walkers launch an attack on Hardhome as the Free Folk board ships bound for Castle Black. Their wight army overwhelms the defenders and sends a panic through the settlement, and wildlings try to swim to the ships instead of boarding rowboats. Those that die are resurrected as wights by the Night's King