William had been delayed for a few weeks by unfavorable winds and weather. The delay turned out to be crucial: had he landed in August when he originally planned, Harold would have been waiting with a fresh and numerically superior force. After landing in England, William's army assembled a prefabricated wooden castle near Hastings as a base.
The choice of landing was a direct provocation to Harold Godwinson as this area of Sussex was Harold's own personal domain. William began immediately to lay waste to the land. It may have prompted Harold to respond immediately and in haste rather than to pause and await reinforcements in London. Again, it was an event that favored William. Had he marched inland, he may have outstretched his supply lines, and possibly have been surrounded by Harold's forces.
They fought at the Battle of Hastings on October 14. It was a close Norman victory but in the final hours Harold was killed and the Saxon army fled. With no living contender for the throne of England to oppose William, this was the defining moment of what is now known as the Norman Conquest.
William was acclaimed King of England around the end of October and crowned on December 25, 1066 in Westminster Abbey.
For more information:
BBC - History - The Norman Conquest
Wikipedia: Norman Conquest of England