Can Anybody Tell Me What A "Moderate Rebel" Is?

I'm watching a frontline episode about ISIS, and they are talking about moderate rebels in Syria.

I dunno, but in my world if you are a rebel against your government (armed rebel, btw), you are by definition EXTREMIST.

Please, someone, explain to me the difference between an extremist rebel, a moderate rebel, and the 3rd time of rebel, which I don't even know what it is!