In case you were wondering about the multiplicity of names...
We may ask what's in a name? But there are issues of political correctness, legitimacy and accuracy at stake: many in the international community -- especially in the Muslim world -- do not want to grant what is clearly a lethal terrorist outfit legitimacy by referring to what it calls itself in Arabic: ad-Dawlat al-Islamia, literally translated as 'the Islamic state'.
It may be helpful to recall what these multiple acronyms actually stand for. While the meaning of IS has been explained above, ISIS is the English translation of what the outfit called itself in Arabic before it transformed into a self-declared 'caliphate' in June of last year: ad-Dawlat al-Islamia fil Iraq was Shaam, or the Islamic State