What is the difference between Colloquial Arabic (spoken Arabic, or aamiyah) and Modern Standard Arabic (written Arabic, or fusha)?
Colloquial Arabic is the spoken Arabic in an Arab country. This is the language that is necessary to communicate with Arabs and is especially important for you to learn if you plan to live in the Middle East. Also, if you plan to work in an Arab country, it is imperative that you know Colloquial Arabic in order to communicate with your colleagues.
Modern Standard Arabic is formal Arabic, or the Arabic you will find in newspapers, books, some films, some television shows, and translations. In addition, if you plan to work in the Middle East, a lot of the work and writing is done in Modern Standard Arabic.