Arab does not equal Muslim. The largest Muslim population is in Indonesia and Malaysia (along with Bangladesh and Pakistan). The largest population of Arabs is in North Africa and the Arabian Peninsula and share Arabic as a common language.
Iran is Persian. The language the majority speaks is known either as Persian or Farsi.
In Afghanistan the majority of the population speaks Pashto. Yeah, that country we've been in for thirteen years doesn't speak Arabic.
In Pakistan, one of the most populated countries in the world, they speak Urdu, an Indic language that sounds so much like Hindi that an Urdu and a Hindi speaker can carry on a conversation with one another without knowing anything about the other's respective language (the written scripts are another story).
The second largest Arab population in the world resides in metropolitan Detroit; largest population of that existing in Dearborn. A large percentage of those Arabs identify as Christian (also other religions and non-religions). Having been to Dearborn I can attest to the fact that the city is not under Sharia Law.
A majority of Muslim women do not wear burkas. That head scarf you've seen on many Muslim women is known as a hijab. Many women who wear hijabs wear more than just the color black. They have many styles from which to choose.
There are many sects in Islam. The three biggest are Sunni, Shia, and Sufism. The major schism in Islam had been over the recognition of the rightful Caliph after the death of The Prophet. Shiites believe that Muhammod's nephew, Ali, was the first rightful leader of the Islamic state. Sunnis believe The Prophet's loyal companion and the first Caliph, Abu Bak'r, to be the first. Thus the first major schism in Islam was political and not religious. The difference in cultural and religious practices came later due to this primary division. Sufis are the more mystical and more liberal of the three and is associated more with Turkey (a country, like the many other countries with a majority Muslim population, I neglected to mention earlier) .
And finally, the number of Muslims who are terrorists is most likely comparable to the percentage of Italians participating in organized crime (i.e., a percent of a percent of the population).
Stuff to think about when trying to pigeonhole a population of one billion instead of listening to bigoted dicks like Bill Maher (that's right, I'm tired of seeing him held up as a paragon of liberalism. I agree with him 95% of the time, but the other 5% is so repugnant that I'd rather hear from a more enlightened voice).
Personal note: Most of the practicing Muslims I have met have been wonderful people. I suspect that those three people senselessly murdered in North Carolina this week were the same way.
(if I misstated anything, please let me know in the comments section).