What does tend it to called as arcane? Xerath seems more like a tiny sun that has high temperature (blue). He burnt out the sun priest when he pulled him to himself.
Before Bloodline, I thought he would turn everything to dust, would destroy until molecules, rather than just burning. Because it sounds like magic, rather than just fire. And arcane is magical energy.
I just wanna hear what is the difference.
Well going with a physics esque answer, one is energy, the other is a chemical reaction. The fire would be as a result of getting close to Xerath, but Xerath himself is not fire. Remember, stars are made of plasma, not fire. Suns don't burn so to speak; they produce energy that in itself emits heat and light. Burning is the physical reaction to getting near that energy, as the body breaks down violently.
Arcane - Meaning "hidden, secret; mysterious, abstruse," it is from Latin arcere, "shut up," from arca, "chest." http://www.thefreedictionary.com/arcane
Taking that in consideration, i guess that Arcane Magic is any kind of magic that is secret and uncomon. Magic that is known and understood by very few people.
Look at others users of "Arcane Magic" like 
, the powers of his magical thing do not look like the powers of Xerath, but both magics are categorized as "Arcane"
So, all the magical misiles of Ezreal, the magical lightings of Xerath, the magical blue fire of Xerath, the magical spells of Ryze, can be categorized as "Arcane Magic" even if those magics do not look the same.
At least that is what i think.
