These were the names for the two most common French dialects in the Middle Ages. The langue d'oc was spoken south of a line running roughly from Bordeaux to Grenoble, and langue d'oïl The names come from the two different words for 'yes' (literally 'language of yes'). The langue d'oc eventually evolved into modern Occitan, a minority Romance language which is for all intents and purposes synonymous with Provençal (though this term is more specific to the region of Provence). This became the language of the troubadours in Southern France. Today the language is spoken by up to 5 million people, but of course France gives it no official status, because the country fears anything that can be seen as a threat to French culture and language.
The langue d'oïl was the language of north and central France, notably of Paris. When Paris on the Ile de France became to the central power in France, the langue d'oïl became the standard for the whole country, and eventually evolved into Modern French. The word oïl 'yes', for which it was named, became oui. Then of course, French spread all over the world, into Canada, Louisiana, Africa, the Carribbean and other places. Thus, langue d'oïl became the modern form of French which is anally upheld throughout the word by the Académie Français, though no one really listens.