You;ve gotten several excellent answers here so far, and I have very little to add that might be helpful. There are plenty of more sophisticated way to talk about this historical event and its ramifications. My students have always found this interesting, though, so I share it with you, as well.
The French-speaking Normans, after the conquest, became the overlords of the Old English-speaking Anglo-Saxons. Language became one indicator of class and social status; speaking French was a signal to others that one had a certain social standing (which is why Geoffrey Chaucer, who wrote The Canterbury Tales in English in order to be accessible to the common man, was villified by his wealthier and higher-class peers.)
One interesting carryover still in our language is the words for animals and meat. The Anglo-Saxon serfs were the tenders of animals meant for the tables of their rich overlords; they rarely saw any of the meat from those animals. The French who lived in the manors rarely saw the live animals but certainly ate the meat they provided. That's why we call the animal "swine" (Old English) but the meat from a pig is called "pork" (French). The same is true for cow and beef, sheep and mutton, hen and poultry...and the list goes on. This principle is undoubtedly true for other areas of our language, but this is the one with which I'm most familiar.
Eventually, the Plague was the great equalizer of both class and language in Britain. When the French landholders were forced by necessity to grant land to the English, the distinct divisions in language became rather a co-mingling--the foundation for the Germanic languages we have today.
Hope you find that helpful--or at least interesting!
The Norman invasion of England in 1066 was perhaps the greatest turning point in the history of the island nation. Not only did the invasion itself and William I's subsequent displacement of the English social sytem in favor of its Norman counterpart alter the course of English history, but the very language itself also underwent a transformation. In the very basic sense, those titles of nobility with which most are familiar only begin to exist in England after the Conquest. "Duke," for example, derives from the Latin word for leader (Dux); there was no such thing as a duke in England before 1066. The very notion of a "county" did not exist before 1066 either.
Besides the Norman linguistic contribution to the very social structure of England, many of the words with Latin or French origin came more and more into use as early as the late eleventh and into the twelfth centuries. Over the course of time, many of the traditional words that derived from Anglo-Saxon language were displaced by their Norman French equivalents. This includes common names. While Anglo-Sasxon names like Edward, Edmund, and Alfred remained somewhat popular after the Conquest (and even up to this day), others like Aethelberts Athelstans, and Egberts were supplanted by Roberts, Rogers, Geoffreys, and Henrys.
In terms of common vocabulary, much of the Norman contribution can be described not by replacement but rather addition. Norman French added to the existing Anglo-Saxon vocabulary, primarily out of political necessity. In order to ensure a smoother transition between the Anglo-Saxon system and the incoming Norman system, William I realized that a complete overhaul of the language was not only unrealistic but also undesirable.
What was known as the Old English language was a combination of borrowed languages with a Scandinavian root. This was due to the strong Saxon presence in Britain prior to the Battle of Hastings in 1066 and the Norman Conquest. Being that Normandy is a territory of France, the infusion of the French language began to exterminate what was known as Old English (Saxon) and it began to replace names and terminonogies because the English responded to the new form of government. In addition to this, since the Old English was not an established language, scribes and other historians did not try in any way to conserve old words. Instead, the blended in the new language and simply killed the old one. To this day we see in words, accents, meanings and even intonation a strong essence of French in the Modern British language.