The Iron Islands are one of the constituent regions of Westeros. The Iron Islands were long an independent nation confined to their islands before they invaded Westeros roughly a century before the War of Conquest, eventually going on to conquer the Riverlands. They were defeated by Aegon the Conqueror and forced to swear fealty to him.
Aegon had to utterly destroy the then-ruling House Hoare during the Conquest, so allowed the ironborn to choose their new ruling house. They chose House Greyjoy of Pyke, who have ruled the islands ever since.
The ironborn are independent, fierce and sometimes cruel. Theirs is a harsh land and they hold no truck with mainland or 'green land' way, which they hold to be soft and womanly. They use horses, who do not breed well on the islands, only as beasts of burden or conveyence, not in battle, and prefer the use of ships. They are raiders and reavers, not farmers. They despise the soft gods of the mainland, preferring their Drowned God who rewards the strong.
Bastards born in the Iron Islands take the surname 'Pyke'.
The Iron Islands are rocky, which makes it difficult for farming. There are mines on the islands, but they do not contain any precious metals- only iron. Iron can be obtained through backbreaking labor. Wealth is spare here, which is why the people of the Iron Islands are known for raiding.
Originally, each island had its own kingdom, with each kingdom having two kings (a rock king of the land and a salt king of the sea). This changed 5,000 years before the Greyjoy Rebellion. Now, one king rules all of the islands.