The rulers of the Ironmen, the warlike seafaring culture of the Iron Islands. The Greyjoys used to make their living by raiding mainland Westeros and are actually proud of pillaging and reaving to survive. As their house words "We do not sow", they take pride in the fact that they perform no honest labor of their own, preferring to steal from others instead. They do not consider themselves members of the Seven Kingdoms, and will rise up at the first opportunity. Their seat is at Pyke and their sigil a golden kraken.
The Iron Islands were settled by the First Men many thousands of years ago. Somewhat isolated from the rest of Westeros, they did not take up the worship of the Old Gods of the Forest, instead creating their own religion based around the Drowned God and the Seastone Chair, which they allegedly found already standing on the shores of Old Wyk when they arrived. When the Andals overran Westeros and conquered the Iron Islands, they intermingled with the natives. Whilst a few locals converted to the Faith of the Seven, it did not fully take hold and worship of the Drowned God continued.
Under the rule of House Hoare, the ironborn reached the zenith of their power approximately four centuries ago. Their head, Harren Hoare (also known as Harren the Black ruled over the western coast of Westeros and enslaved the peaceful people of the Riverlands, forcing them to build a magnificent fortress known as Harrenhal, a project that took three generations to complete, draining the Riverlands to finance the building, with thousands of captives from around dying in the quarries chained to sledges.
Harren had finally completed his grand castle of Harrenhal, when Aegon the Conqueror landed in Westeros. Harren's tyrannical rule over the Riverlands earned him little love from his lords. When Aegon swept through the region, many of the river lords, led by Edmyn Tully, revolted against Harren to support the Targaryen invader and joined the Conqueror's host. When Aegon the Conqueror unleashed his dragons, who were not obstructed by the towering walls of Harrenhal and roasted Harren and his sons in the tallest tower of the castle, ending his rule and kingdom. Harren was the last king of the Iron Islands, after his death Lord Vickon of House Greyjoy was given dominion over the Islands in the name of the Iron Throne.
After the War of Conquest the ironborn were driven back to their islands. The ironborn were subjects of the Seven Kingdoms and the king's peace for almost three hundred years, until nine years after the War of the Usurper, when Robert Baratheon toppled the Targaryen dinasty and established his own rule. The ironmen revolted in an attempt to restore the Old Ways led by their overlord Balon Greyjoy.
Though they manage to perform successful early raids against the west coast of Westeros, the Iron Throne retaliated having superior forces numbers and resources at their disposal. Robert launched a full attack with all the lords of Westeros in his commands and they crushed the rebellion completely. Balon was forced to bend the knee and swear fealty to the Iron Throne once again, while his surviving son Theon was sent into the care of Lord Stark as a hostage to ensure Balon's good behavior.
Though defeated, Balon Greyjoy never gave up on his desire to become King of the Iron Islands, at the out break of the War of the Five Kings, an opportunity presented it self. Balon assembled his fleet and ordered all ships which visited the Iron Islands to remain so he would have the element of surprise.
Robb Stark, the King in the North, had offered to ally with Balon against the Lannisters but Balon spurned the offer, which was carried to him by his son Theon. Instead he declared himself King of the Iron Islands and the North, taking the Neck and cutting off Robb's army campaigning in the Riverlands and the Westerlands. Then the ironborn, led by Asha, Theon, and Dagmer Cleftjaw, went ravaging the western coastline of The North.
His forces were able to conquer a number of castles because most soldiers had left with Robb to fight in the south. Amongst these were: Deepwood Motte, Torrhen's Square and the seat of the Starks: Winterfell.