On October 14, 1066, William the Bastard defeated King Harold at the Battle of Hastings, conquering England. The battle changed English history and marked a close to the Viking age that had begun almost three centuries earlier.
To give some background, the first and most famous Viking raid into England was the 793 A.D. raid on the abbey at Lindisfarne. Less than a century later, the Vikings had come to stay. They held sway over half of England and were vying with Alfred the Great for total control. By the year 1016, the Viking king Cnut achieved the conquest of England.
To the south, the French under King Charles the Simple surrendered to the Vikings, ceding to them that portion of France known as Normandy in 911. It was from Normandy in 1066 that Duke William launched his attack on England.
On October 14, 1066, Harold's army occupied the high ground at Senlac Hill near Hastings. His army was composed entirely of infantry who relied upon the ancient “shield wall” tactic.
At the base of the hill was William's modern army, composed of archers, infantry and cavalry.
William opened up the battle with a barrage of arrows followed by an infantry assault. The shield wall of Harold's army held strong, stopping the Norman advance. William launched his cavalry only to see them beaten back. When a portion of the Norman army fell into a route, a sizable portion of Harold's army broke the shield wall and charged after the retreating Normans. William led a group of his cavalry in a counterattack, decimating the pursuers. With the sizable portion of his army lost, Harold's defeat was inevitable. Soon thereafter, a Norman arrow pierced Harold through the eye, killing him. With their leader dead, Harold's army disintegrated and William claimed a decisive victory. He would spend the next decade consolidating his hold on all of England.
The story of the battle of Hastings is memorialized in perhaps worlds most famous tapestry – the Bayeux Tapestry, some seventy feet long and containing over 50 scenes. The one below shows the death of King Harold.
The Norman occupation of England was brutal. William treated the rebellious and the holdouts with ruthless ferocity. Virtually all Anglo-Saxons were stripped of their land as William redistributed it to his knights. William went on a building spree, erecting countless castles throughout Britain from which his knights could defend their holdings. William co-opted the sophisticated English forms of government, but within ten years, had replaced all native English with Normans. The Norman "yoke" would last over a century, gradually dying out as Normans and native Anglo-Saxons intermingled and married, coupled with the 13th century loss of Norman territory in France. His reign also saw the decline of slavery as a practice in England, though that occurred because slavery was becoming economically inefficient.
Update: The UK's Telegraph offers its own retrospective on the legacy of William's victory at Hastings: In everything we say, an echo of 1066.
William introduced many innovations from France, including numerous French words that would become part of the English language. And his reign would set the stage for French and British enmity that would follow down the centuries.
One other thing of note from William's reign was the creation of the Doomsday Book - a massive survey undertaken in 1086 to document all landholders and their material worth for the purposes of taxation. It was called the Doomsday Book because the decisions of the assessors were not appealable. It's importance today as a historical document giving a comprehensive view of Medieval England cannot be overstated.