The heart of the Arab world is made up of the six founding members of the Arab League--Egypt, Iraq, Jordan, Lebanon, Saudi Arabia and Syria. The six forked the Arab league in 1945. Other Arab nations in the Middle joined the League as they won their independence or were voluntarily drafted into the non-binding alliance. These include, in that order, Yemen, Libya, the Sudan, Morocco and Tunisia, Kuwait, Algeria, the United Arab Emirates, Bahrain, Qatar, Oman, Mauritania, Somalia, Palestine, Djibouti and Comoros.
It's arguable whether all people in those nations consider themselves Arab. In north Africa, for example, many Tunisians and Moroccans consider themselves distinctly Berber, not Arab, although the two are often considered identical. Other such distinctions abound within various regions of the Arab world.