The word "Arabic" also refers to the many national or regional dialects/languages derived from Classical Arabic, spoken daily across North Africa and the Middle East, which sometimes differ enough to be mutually incomprehensible. These dialects are not frequently written, although a certain amount of literature (particularly plays and poetry) exists in many of them, notably Lebanon and Egypt.
"Colloquial Arabic" is a collective term for the spoken languages or dialects of people throughout the Arab world, which, as mentioned, differ radically from the literary language. The main dialectal division is between the Maghreb dialects and those of the Middle East, followed by that between sedentary dialects and the much more conservative Bedouin dialects. Maltese, though descended from Arabic, is considered a separate language. Speakers of some of these dialects are unable to converse with speakers of another dialect of Arabic; in particular, while Middle Easterners can generally understand one another, they often have trouble understanding Maghrebis (although the converse is not true, due to the popularity of Middle Eastern, especially 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 fiih, and North African kayen all mean "there is", and all come from Arabic (yakuun, fiihi, kaa'in respectively), but now sound very different.
The major groups are:
Egyptian Arabic (Egypt) Considered the most widely understood and used "second dialect"
Maghreb Arabic (Tunisian, Algerian, Moroccan, and western Libyan)
Hassaniiya (in Mauritania)
Andalusi Arabic (extinct, but important role in literary history)
Sudanese Arabic (with a dialect continuum into Chad)
Levantine Arabic (Syrian, Lebanese, Palestinian, and western Jordanian)
Gulf Arabic (Gulf coast from Kuwait to Oman, and minorities on the other side)