The Norman Conquest of England in 1066 forever wedded two cultures (Norman/French) and Germanic/Welsh,Celtic/Scotish/Irish of England and the surrounding areas. Today, you can see clear indications of French and Germanic influence in the modern English language. Some English words were replaced with French and eventually some of the respective grammatical rules blended. Also, the Normans were “northmen” Vikings who settled in France; so they too were a cultural mix of Norsemen and French.
William conducted a survey to determine taxation, called the Domesday (Doomsday) Book. The English aristocracy was replaced. The Normans created a centralized control over the relatively efficient system of English ‘shires’ which were autonomous and mostly self-governed cells. William and his successors were vassals to the King of France, but as King of England, William was a pier with the French King, which led to a tenuous relationship and established a continuing rivalry between England and France.
The most obvious difference is in the language. English prominence in its own aristocratic rule did not return until the 14th century and it would not be until the 15th century that they stopped speaking French and started speaking what was then Middle English in the courts.
Check out the links for a comparison between Old, Middle and Modern English.