Sources are the very base elements in which magic takes form. The sources listed below are universally accepted as the only elements that magic users can take control of.
Most Spellcasters are tied to only one source which is revealed when they first begin to show signs of magical ability. Cases of spellcasters being attuned to more than one source is not unheard of, but exceedingly uncommon. Still, such a limiter is hardly a hassle to the creative spellcaster. For example, creating an intense wind could allow an air-attuned spellcaster to carry water or nurse a fire into an inferno.
All sources have their uses, though it is noted that some are more effective overall depending on a spellcaster's Sign.