Latin was introduced to Gaulish-speaking France (then called Gaul) when Julius Caesar conquered the region in 51 BC, and eventually it became the common language of the region. In the 4th century the Franks came to rule northern France. They spoke Frankish, a Germanic language, but eventually the Vulgar Latin of the common people dominated.
After this point the Latin of France can be divided in two: the langue d'oc of the south, which was the ancestor of Occitan; and the langue d'oïl of the north (which had more Frankish influence), which can be said to be Old French.