Vulgar LatinVulgar Latin is a blanket term covering the vernacular dialects of the Latin language spoken in the vast provinces of the Roman Empire starting from the second and 3rd century CE, until its direct merging with the early romance idioms.
The name "vulgar" simply means "common": it derives from the Latin word "vulgus", meaning "people".
By the strictest definition, Vulgar Latin was a spoken language, "late" Latin being used for writing (the general style being a bit different from the "classic" standards, usually considered as referred to texts of first century AD).
Vulgar Latin developed differently in the various provinces of the Roman Empire, thus gradually giving rise to modern French, Italian, Spanish, Portuguese, Romanian, Catalan, etc. Although the official language was Latin, Vulgar Latin was what was popularly spoken until the popular language finally turned to "proper" localised forms. Obviously Vulgar Latin is considered lost when the local dialects start collecting enough local characteristics to form a different idiom. They evolved into romance languages when an independent value was recognisable in them (eg. Oil, Oc, Sì). The word "Romance" comes from "romanicae loci", of a place in the Roman Empire; this still ideally includes all the dialects as a part of the whole Latin family.
The 3rd century AD is presumed to be the age in which, apart from declensions, many roots were changing (i.e., "equus" → "caballus", etc.). Recently, some studies (which still perhaps need more scientific development) have suggested that pronunciations too started to diverge, supposedly with already a similarity to modern local pronunciations, with the most spectacular (alleged) effect in the area of Naples. However, these changes were obviously not uniform in the Empire's territory, so the greatest differences were perhaps to be found among different forms of Vulgar Latin in different areas (also due to the acquisition of newer "local" roots), even if it should be noted that most of theory is based on reconstruction a posteriori rather than, evidently, on texts (poor people could use poor supports, of which poor remains could last for a direct knowledge in our age).
At the third Council of Tours in 813, priests were ordered to preach in the vernacular language -- either the "rustica lingua romanica", Vulgar Latin now recognizably distinct from the frozen Church Latin; or German -- to be comprehensible. This could be a documented moment of the evolution. Late Latin, still based in Rome, presumedly reflected these acquisitions, recording what was changing in a nearer area - fairly identifiable with Italy. Formal Latin was then "frozen" by the codifications of roman law on one side (Justinian) and of the Church on the other side, finally unified by the medieval copyists and since then forever separated from already independent romance vulgar idioms.
Vulgar Latin is then a collective name for a group of derived dialects with local - not necessarily common - characteristics, that don't make a "language", at least in a classical sense. It could perhaps be described as a sort of "magmatic" undefined matter that slowly locally crystallized into the several earlier forms of each Romance language, that consequently find their ultimate proper ancestry in formal Latin. Vulgar Latin was therefore an intermediate point of the evolution, not a source.
Vulgar Latin should not be confused with Pig Latin.