Arabic is one of the most widely spoken languages in the world, with an estimated 290 million speakers across the board.
Except not really.
Arabic language is in reality more like a collective of languages, spoken in different ways in different parts of the Arab world. Like Germanic or Slavic languages, Arabic languages come in a variety of shapes and colors while retaining some core characteristics, such as sounding pretty much the same to foreigners.
Historically (well, until 630 AD or so,) Arabic language and its mostly similar tribal dialects were limited to the Arabian peninsula, expanding and evolving within it. Non-Arabs in the middle east spoke their own indigenous languages; for example, North African Berber nomads spoke Berber languages. Until the Spread of Islam, that is, after which Arabic gradually replaced the indigenous language as the main language for governance and everyday life on the land, a process accelerated by Arabian (Bedouin) nomads migrating out of the Arabian peninsula and into North Africa, Mesopotamia and the Levant.
In non-Arabian areas ruled by Arabian chiefs, creoles of Arabic and the local language formed, creating what is today known as Arabic dialects, but like the relationship between Haitian Creole and French, distant dialects of Arabic are not very mutually intelligible. Having developed separately out of different combinations of languages, Arabic dialects have different vocabularies and accents (that are not necessarily mutually intelligible), each dialect effectively being a language in its own right. Some dialects are simple enough to be easily understood by non-native speakers, though (Egyptian and Levantine, for example), and geographically proximate dialects tend to have a lot in common.
The explanation for lumping such radically different creoles in the same pack is largely sociopolitical. Classical Arabic is a variety of Arabic that is no longer spoken at home but is understood by most educated Arabs (think Old English if it was taught in schools), and is the one used in writing the Qur’an, the Islamic holy scripture, back in the 7th century. The Qur’an played a great role in maintaining the status of Classical Arabic, which would have gone extinct if the Qur’an hadn’t existed. The study of Arabic language developed and became popular (even with non-Arabic speakers), and this preserved the status of Classical Arabic and contributed to its reputation with Arabs (or, speakers of Arabic creoles).
The other reason is Arab nationalism. As a response to colonial exploitation, Arabic peoples eventually came to see “Arabism” as a uniting banner (compare to this, or this). Needless to say, Arabic language rather than culture was the main uniting aspect. Pan-Arabism gained a respectable amount of traction in the 50s, becoming the state ideologue of quite a few Arab states, and eventually inspiring its “same but different” sister ideology: Pan-Islamism. Both are mainstream across the Arab world today, to varying levels across different communities.
All research credit goes to Wikipedia. Please let me know if there are any mistakes in this post.