The history of the French language begins with the invasion of Gaul by Julius Caesar's armies in 59 B.C. The land was then inhabited by a multitude of different tribes who spoke various related Celtic languages. Subsequent to the conquest of the territory by 51 A.D., however, the language of the Romans was gradually adopted by most Gauls over the next few centuries. The Latin spoken by the invaders was not the careful, cultivated form of Latin used in the Roman Senate and in literature, but rather, a completely oral form of the language, complete with its own grammar, vocabulary and pronunciation patterns. Once established and adopted in Gaul, the language naturally changed rapidly, particularly after the fall of Rome. It is interesting to note that language of the Gauls had practically no influence on the Latin vernacular that subsequently developed. Beginning in the fifth century, the land was subject to numerous invasions--Germanic tribes from the east (the Franks) and the Vikings from the North. Each left only minor influences on the language. The invasions did, however, serve to accelerate a growing division between the language spoken to the south of the Loire--Langue D'Oc --from that spoken in the North, Langue D'Oïl (Oc and Oïl are the words for 'yes' in their respective dialects). The Southern dialect remained close to its Roman roots, while the Northern dialect showed exterior influences. In 987, Hugues Capet was elected king of the small kingdom of Île de France, centered around Paris. Although France at this time was essentially composed of small, independent kingdoms, this event marked the beginning of political unity, and therefore, of linguistic unity. Capet was the first king in the territory to speak the vernacular, and as his kingdom grew larger and more powerful through his successors, so too did the prestige of its language (called Francien today).
Linguistically, the development of Old French from Vulgar Latin is distinguished by complex phonetic transformations and a simplification of the Latin case system. By the time of the Serments de Strasbourg (842 A.D., see Earliest French Texts Page) there were just two cases in French, a subject case (cas sujet) and an oblique case (cas régime). By the end of the middle ages in France (with the invasion of Italy in 1496), the two-case system would have been defunct for a century. During this period, French differed considerably from region to region, as shown by the various different written texts that have come down to us.
Wikipedia's Entry on "Old French" 
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