Latin
was spoken too widely in Europe, Africa and Asia. It was carried there
constantly by Italian colonists who brought their tongue with them and
assimilated gradually aboriginal nations. That is how linguistic mixtures
appear. But it is not the way Popular Latin appeared first. Classical Latin
began to suffer changes in colloquial language already in the last centuries
BC, when Rome finally managed to conquer all Italy. The Italics, who spoke
Umbrian, Oscan, Volscian
etc., mixed slowly with Latin colonists. This caused serious changes
in the spoken language. The process of simplification and analitization
began at that time. Endings of nouns and verbs began to be dropping everywhere,
phonetics began changing a little, and even the dictionary acquired more
new words with parallel loss of previously used. Some progressive features
of Umbrian became now a part of Latin.
But the flourishing epoch of Popular Latin began when Roman colonists faced Celtic, Germanic, Thracian, Illyrian and Iberian lands and peoples. Here mixing with so various nations turned Latin into varieties of dialects, and it's them that are called Popular Latin. It existed on vast territories of Europe since the 2nd century and till the 6th century AD, until it turned into national Romance languages: Old French (Langue d'oil), Old Provencal (langue d'oc), Spanish, Portuguese, Rumanian, Rumantsch etc. Nowadays Romance languages are numerous, with numerous dialectal groups, but the source was only one - Popular Latin.
Great phonetic changes were one of the main features of Popular Latin. New sounds appeared (like [dj], [j], [sh], [ch], sibilants, aspirants, dentals like [th], [ts]), diphthongs almost disemerged, final consonant dropped practically everywhere.
Such phonetic revolution led to grammar changes. Popular Latin carried a tendency to an uninflected language. From the six Latin noun cases, only 3 or 4 (in pronouns) remained here, and afterwards even 2 cases, for other cases were expressed simply by prepositional nouns - the way it is done now in English and French. The definite (from demonstrative pronoun ille) and indefinite (from numeral unus) appeared, as well as the 3rd person personal pronouns which Latin lacked. They also were formed from demonstrative pronouns. The verb system, so terribly complicated in Latin, was altered, and since then auxiliary verbs started expressing perfect, preterite, plusquamperfect and future forms. The only thing that was preserved was the infect verbal endings.
Popular Latin with all its varieties has much in common everywhere among
Romance languages, but the dialectal features depend on the region, on
the nation which used to live here before Romans came and assimilated it.