Within the Arabic speaking world there is considerable dialect diversity. In addition to the colloquial dialects, there is a superposed literary language which can be roughly divided into three historical periods: (1) Classical Arabic, approximately spanning the sixth through eighth centuries, characterized by the dominant influence of Arabs themselves (politically and culturally, including the literary output), and most notably including pre-Islamic poetry, the Koran, and the early Islamic literature; (2) Medieval Arabic, continuing up through much of the Nineteenth Century, characterized by a codified grammar and the predominant influence of non-Arab writers, and culminating in a long period of declining social and political importance; and (3) Modern Written Arabic, reflecting a renaissance in Arab self-consciousness and certain very notable trends toward breaking with the traditional forms and ideals.
Thus, in the Arab world today, Modern Written Arabic has no native speakers, is rather specialized in function as the vehicle of literary expression within Arab society, is of high prestige as the modern counterpart of a highly esteemed literary language of thirteen centuries duration, and has had a long history of painstaking, thorough grammatical study. It has, in addition, a number of grammatical categories not present in the colloquials.
Resource: Foreign Service Institute Modern Written Arabic