Colloquial Arabic is a collective term for the spoken dialects of Arabic used throughout the Arab world, which differ radically from the literary language. The main dialectal division is between the varieties within and outside of the Arabian peninsula, followed by that between sedentary varieties and the much more conservative Bedouin varieties. All of the varieties outside of the Arabian peninsula (which include the large majority of speakers) have a large number of features in common with each other that are not found in Classical Arabic. This has led researchers to postulate the existence of a prestige koine dialect in the one or two centuries immediately following the Arab conquest, whose features eventually spread to all of the newly conquered areas. (These features are present to varying degrees inside the Arabian peninsula. Generally, the Arabian peninsula varieties have much more diversity than the non-peninsula varieties, but have been understudied.)
Within the non-peninsula varieties, the largest difference is between the non-Egyptian North African dialects (especially Moroccan Arabic) and the others. Moroccan Arabic in particular is hardly comprehensible to Arabic speakers east of Libya(although the converse is not true, in part due to the popularity of Egyptian films and other media).
One factor in the differentiation of the dialects is influence from the languages previously spoken in the areas, which have typically provided a significant number of new words and have sometimes also influenced pronunciation or word order; however, a much more significant factor for most dialects is, as among Romance languages, retention (or change of meaning) of different classical forms. Thus Iraqi aku, Levantine fīh and North African kayən all mean ‘there is’, and all come from Classical Arabic forms (yakūn, fīhi, kā’in respectively), but now sound very different.