For friends interested in learning Arabic.

You should know that Arabic speaking communities experience what linguists call "diglossia" http://en.wikipedia.org/wiki/Diglossia

The language spoken natively by the people and used in daily life is NOT the Standard Arabic.

We don't actually speak with Standard Arabic, and it sounds strange to us .. to hear someone speaking in it and expecting us to reply in Standard Arabic.

This has a few implications:

1. The "Standard Arabic" is not anybody's native langauge. Native Arabic speakers are likely to make mistakes when trying to speak or write in Standard Arabic.

2. There are no "slang" expressions in Standard Arabic. All slang expression by definition are dialects. For instance, there's no equivalent in Standard Arabic for "Hey, what's up!". If someone gives you an expression to say that in Standard Arabic, it will probably be the equivalent of "Hello, what are the news?", which is much more polite and not so slang.

3. Standard Arabic is useful for reading books and newspapers, but it's not so useful for watching tv shows or listening/talking with Arabs in how they speak naturally.

To make things more complicated, there's also "Classical Arabic". Now, Arabs don't distinguish between Modern Standard Arabic and Classical Arabic, however it appears that linguists do. I think if you only learn the "Modern" Standard Arabic, no matter how advanced, you will have a difficult time understanding literature written in Classical Arabic, including but not limited to: the quran, classical poetry, old books.

Actually, most Arabs have difficulties with these. I tried to read books written by Ibn Sina (Avicenna) and found it difficult. I had to read slowly and take notes. It might be as bad as trying to read Shakespear for today's native English speakers.

Classical Arabic has more vocabulary (or you can say: modern use of Standard Arabic tends to confine itself to a subset of the Classical Vocabulary). Also, classical literature uses different  sentence forms and different ways to express ideas.

So, what should you learn? Classical Arabic? Standard Arabic? Dialects?

Well it all depends on what your goals are and what would you like to be able to achieve.

- If you want to be able to speak with native Speakers, you should learn one or two popular dialects. The two most popular dialects are Egpytian (Cairo) and Syrian (Damascus). I personally recommend learning Syrian dialect first. It sounds nicer :) but this is just my personal taste. But either way, all Arabs in the middle east can understand both these dialects.

- If you want to be able to read and write for political or business popruses, you should learn Modern Standard Arabic. It's the language of politics and business. You can also use it to communicate with Arabs in general. Even though it doesn't seem natural to us, we can and do communicate using Standard Arabic. Many serious Arabic forums on the internet use Standard Arabic. I mean that people in these forums tend to communicate using Standard Arabic. Though entertainment based forums (including pop culture) tend to use dialects.

- If you're mostly interested in religious studies (quran, hadith, etc) you should learn Classical Arabic. As I said, Arabs don't actually distinguish between classical and modern standard arabic, but what I mean is, don't waste your time trying to read modern novels or use modern books as studying/learning material. Classical books use a different style and use a slightly different set of vocabulary.

A generalistic approach (which I would recommend for serious learners) is to study both Modern Standard and a popular dialect at the same time.

Because honestly, when Arabs communicate among each other, we use a mixture of dialect and standard Arabic. We use the standard because then we know that others can understand, and use dialect because it feels more natural to us. So if you want to understand us, you should be learn both: the standard and a dialect or two.

This is also useful if you're interested in Arabic Music, because some songs use Standar Arabic while others use dialects.