Medieval Arab genealogists divided the Arabs into three groups:

the "ancient Arabs", tribes that had been destroyed or vanished, such as Ad and Thamud; they are often alluded to in the Qur'an as examples of God's power to destroy wicked peoples.
the "Pure Arabs" of South Arabia, descending from Qahtan. The Qahtanites (Qahtanis) are said to have migrated the land of Yemen following the destruction of the Ma'rib Dam (sadd Ma'rib). The Qahtanite Arabs were responsible for the ancient civilizations of Yemen, notably including that of the Sabaeans (known in the Bible as Sheba.)
The "Arabized Arabs" (musta`ribah) of North Arabia, descending from Adnan, supposed to be a descendant of Ishmael (Ismail), the eldest son of Abraham and Hagar.
The Arabic language as it is spoken today in its classical Quranic form was the result of a mix between the original Arabic tongue of Qahtan and the northern Arabic which borrowed from other northern Semitic languages from the Levant.

In Jewish and Christian traditions, the identification of the Ishmaelites, described in the Bible as a people of the Arabian wilderness, with Arabs began at least by the time of Josephus, and became standard in the early days of Islam (in which the term "Hagarenes", a pun on the Arabic muhajir and the name of Hagar, was commonly used.) Efforts to reconcile the Biblical and Arab genealogies later led to the identification of Joktan with Qahtan, probably due to his Biblical identification as the ancestor of Hazarmaveth (Hadramawt) and Sheba.

Etymology | Traditional genealogy | History | Religions | Who is an Arab?