The Iron Throne is the throne of the Seven Kingdoms, upon which the king rules. It is often used as a metonymic device to refer to the king's authority. Individuals who act on the king's behalf, such as his Hand or regent, may sit on the throne in his absence. The chair itself is cold and hard, with many jagged edges.
Aegon I had the throne constructed at the beginning of his reign. It is forged from the swords surrendered by his enemies. It took 59 days and the fires of Balerion the Black Dread to hammer 1,000 swords into the shape of a throne. The blades are still sharp, and those who sit the throne will cut themselves if they are not careful. This was Aegon's desired effect, as he believed that no king should ever sit comfortably on his throne. It is said that the throne itself has caused the deaths of several people.
The Throne Room
Since the construction of the Red Keep, the Iron Throne is located on a high platform in the Throne Room. Usually, the members of the Kingsguard stand guard below. The room has high narrow windows.
During the rule of the Targaryens, the Throne Room was decorated with the skulls of their dragons. When the Baratheon line overthrew them, the dragon skulls were substituted with hunting tapestries. These were taken down after Robert's death.
It was in the Throne Room where Kings Maegor and Aerys II were murdered. Also, the execution of Rickard and Brandon Stark took place there. Aswell as Joffrey I was poisoned at his wedding feast which took place in the Throne room.
Once Aegon I had conquered six of the seven kingdoms, he proclaimed himself king of Westeros, the rule of the Iron Throne extended over the continent. He was acknowledged by the previous kings in the North, and the Rock, and had the support of the rulers he had appointed to the Stormlands, the Riverlands and the Reach.
The first years of Targaryen reign were a period of uneasiness and turmoil. After Aegon's death thirty-seven years after landing on Westeros, he was succeeded by Aenys I, who was considered a weakling. After him, came Maegor the Cruel, who tried to suppress all opposition to the Targaryen rule taking harsh measures against his opponents. Eventually he was murdered, and the crown was inherited by Jaehaerys the Conciliator, who did some concessions and reached lasting agreements with the contending factions. With his reign, that lasted more than half century, the Seven Kingdoms developed cohesion and the rule of the Iron Throne was consolidated.
The next king, Viserys I, reigned during a time of peace and plenty. But war came upon Westeros after his death when his younger son Aegon disputed the rights of his elder daughter and designated heiress Rhaenyra. Many lords and nobles supported Aegon on the grounds that the Andal laws of succession (favoring males over woman) should be followed. The conflict led to a civil war that would be known as the Dance of the Dragons.
The civil war ended with Rhaenyra's defeat and execution, but the rule of Aegon II was short. After his death, Rhaenyra's son Aegon III took the throne taking the daughter of Aegon II as a wife. Although the conflict had been resolved and the continuity of the Targaryen line was again assured, the war caused great damage to their power: many dragons had died during the fratricidal fightings, thus depriving them of their most valuable resource.
When King Daeron I ascended to the Throne he announced his intention to complete the work of Aegon the Conqueror and invade Dorne. His campaign did manage to invade Dorne, but the rebellious Dornish made holding it a costly adventure. Daeron himself died while trying to solidify control of the area. After him, his brother Baelor proved a peaceful king.
Viserys II came next, but his reign only lasted a year. His son Aegon would end being remembered as Aegon the Unworthy, in part because after legitimatizing his bastards on his deathbed planted the seeds of the Blackfyre Rebellion.
After Aegon came Daeron the good who brought Dorne peacefully into the Seven Kingdoms through a duel marriage pact. He survived the first of the Blackfyre rebellions which ended with the death of his half brother Daemon Blackfyre.
Daeron died in the great spring sickness and was succeeded by Aerys I, a bookish man who left most of the running of the realm to his Hand of the King Brynden Rivers, another Blackfyre rebellion was ended during his rule at Whitewalls. Aerys death without issue let to the crowning of his younger brother Maekar I who ruled a dozen years. He was followed by his son Aegon V who became known as the unlikely seeing as he was the fourth son of a fourth son. House Blackfyre was finally exterminated during his reign in the War of the Ninepenny Kings. Aegon perished in the tragedy of Summerhall.
Next came the sickly Jaehaerys II though frail he was wise and ruled well in his short three years. He was followed by Aerys II who would become known as the Mad King, his madness led to the War of the Usurper and an end of the Targaryen hold on the Iron Throne. Aerys was cut down by one of his own knights who became known as the Kingslayer. Robert Baratheon was crowned King.
House Baratheons hold on the iron throne did not last long either as unknown to Robert his heirs were not his, but bastards born of incest between his queen Cersei Lannister and her brother The Kingslayer. Robert was assassinated by his queens kin and the bastard Joffrey Baratheon took the throne. His claim was not accepted though by the rest of House Baratheon and both the brothers Stannis Baratheon and Renly Baratheon made claims on the throne, while other noble Houses wished to seek independence from the rule of the Iron Throne, this led to the War of the Five Kings.
After two years of bitter fighting the war of the Five Kings was unofficially ended with Joffrey as the victor. Despite his victory however he was poisoned at his own wedding feast and his younger brother Tommen Baratheon was crowned in his place. This child king is the current occupant of the Iron Throne.