The Greyjoys of Pyke claim descent from the Grey King of the Age of Heroes. Legend says the Grey King ruled not only the western isles but the sea itself, and took a mermaid to wife.
For thousands of years, raiders from the Iron Islands, called "ironmen" by those they plundered, were the terrors of the seas, sailing as far as the Port of Ibben and the Summer Isles. They prided themselves on their fierceness in battle and their sacred freedoms. Each island had its own "salt king" and "rock king." The High King of the Isles was chosen from among their number, until King Urron Redhand murdered the other kings and made the throne hereditary when they assembled for a choosing. Urron's own line was extinguished a thousand years later when the Andals swept over the islands. The Greyjoys, like the other island lords, intermarried with the conquerors.
The Iron Kings extended their rule far beyond the isles themselves, carving kingdoms out of the mainland with fire and sword. King Qhored could truthfully boast that his writ ran "wherever men can smell saltwater or hear the crash of waves."
In later centuries, Qhored's descendants lost the Arbor, Oldtown, Bear Island, and much of the western shore. Still, come the Wars Of Conquest, King Harren the Black ruled all the lands from the Neck to the Blackwater Rush. When Harren and his sons perished in the fall of Harrenhal, Aegon Targaryen granted the riverlands to House Tully, and allowed the surviving lords of the Iron Islands to revive their ancient custom and choose whom should have primacy among them. They chose Lord Vickon Greyjoy of Pyke.
The Greyjoy sigil is a golden kraken upon a black field. Their words are We Do Not Sow. The significance of these words is buried in the Ironborns' history of raiding. The Iron Islands are hard and stony places, a hard place that breeds hard people. The soil is thin and poor and there are scant few crops that can grow there. Fishing and tending goats and sheep are the other manners of getting food. The only other way of making a living is mining, for iron, lead and tin, the only treasures of the Iron Islands. In the past, all these things were done by captives captured on the Ironmen's raids on other lands. The Drowned God worshipped by the Ironborn, as told by their priests, made them to reave and rape, fight and plunder, carve out kingdoms and write their names in fire, blood and song. They lived by the axe and sword and took what they will, be it women or wealth. They reaped, but did not sow.
Barely five years after Robert Baratheon took the Iron Throne, Lord Balon Greyjoy of Pyke called his banners in rebellion against it, and proclaimed himself King of the Iron Islands, perhaps so that the Ironborn could once again return to their Old Ways. Robert Baratheon and Eddard Stark had put down the rebellion, and Lord Greyjoy had to bend the knee to the King in surrender and hand his last son Theon to Lord Stark as hostage and ward for his good behaviour.