First, at the most basic level, people need to know that ISIS arose in Iraq (or at least gained much of its momentum) in response to a paranoid crackdown on the majority Sunni population by the U.S.-backed Shia Prime Minister Nouri al-Maliki. Long story short: he created his worst nightmare. ISIS involvement in Syria is partly the result of similar Shia-leadership v. Sunni majority faultlines. In this sense, the conflict has vague similarities to the history of Catholic-Protestant violence in Ireland, and is part of the reason some commentators say the current conflict is "more political than religious." Photo of IRA militants below is from dailymail.uk
I recommend Woods' Atlantic article. Read it. It's helpful. But if you're willing to make that commitment, go the extra mile and read two more. I can't recommend Woods' article without recommending a couple of others.
First, read John A. Azumah's article "Challenging Radical Islam" from the Christian journal First Things. Azumah does a great job of placing ISIS in (or outside of, as the case may be) the very particular traditions of Sunni Islamic law. He indirectly raises a question for Christians: Do we WANT Islam to be fundamentally and uncontrollably violent because that makes us feel religiously superior? I often find that atheists dialoguing with Christians insist on maintaining the most literalistic fundamentalist interpretation of the Bible and Christianity and I think Christians do something similar with Islam sometimes.
Second, read "What is Islamic?" by Haqiqatjou and Qadhi over at MuslimMatters.org. I don't agree with everything they have to say (they have 21 points of response to Woods), but they expand on the details of Islamic jurisprudence, offer some helpful insights (and links) and ask additional helpful questions.