The Greyjoy Rebellion was a rebellion led by Balon Greyjoy against the Iron Throne in 289AL, nine years before the beginning of A Game of Thrones.
Balon Greyjoy wished to secure independence of the Iron Islands, naming himself "King of the Iron Islands", but King Robert I Baratheon would not stand for it.
Balon Greyjoy led several attacks against the Iron Throne. He burned the Lannister fleet and staged an attack at Seaguard, by his son Rodrik Greyjoy, who was killed by Jason Mallister.
With help from his brother Stannis Baratheon, Master of Ships, Robert I Baratheon was able to get a fleet large enough to smash the Iron Fleet. This helped him to get to the Iron Islands.
Robert I Baratheon and Eddard Stark led the final battle at Pyke, the seat of House Greyjoy. Robert attacked the southern wall, eventually gaining access to Pyke. There, Malon Greyjoy, another of Balon Greyjoy's sons, was killed. Jorah Mormont earned his knighthood that day, for his good work in battle. Theon Greyjoy, Balon Greyjoy's nine-year-old only surviving son was taken as a hostage in House Stark, to ensure good behavior by Balon. However, as time went on Theon became part of the Stark family and began to see the other children as his siblings, especially Robb Stark.