system
Given the following post, respond as if you're commenting on Reddit:
human
[Warcraft] How does magic work?

From a spellcasters point of view how does magic work for the "wizard" classes (i.e. mage, warlock, priest, druid to an extent).

Shamen seem to be granted their magics from spirits of nature, druid's have a similar deal going on, but the balance discipline seems to veer into wizard territory.

But of the 'wizard' classes how does the magic work? They shape their mana into spells and release it, but where do these spells come from? Are they incantation based? Is there a "weaving" of their mana into spells? What's the difference between a warlock casting Incinerate and a mage casting Fireball? How do the elements of fire, frost, arcane, holy and shadow relate to the same mana resource innate to the caster?

Is it simply usage that separate a mage, priest and warlock, or are they distinct magic systems?
gpt
The literary examples of how Magic works indicates its something like a circuit. Magic users basically take energy (Arcane being the base, natural magical source. Everything else is a variation of it)and through a combination of gestures and sheer will, shape it into patterns which elicit particular results. 

Different types of magical energy (basically wavelengths) have different reactions when harnessed in the same way, and depending upon what they are used. Fel and Holy tend to cancel out violently, somewhat like Matter and Antimatter,  while Holy and Shadow only nullify each other. 

In terms of general practice... Most spells are simply memorized patterns that a magic user can easily put energy into, and eliciting a predictable effect. More complicated spells require more time and effort, often using physically reagents and focus points in complex circuits (generally referee to as the Spell Matrix) but incantation is rarely, of eger necessary. It can help novices focus, mind you.