Mana is that which defines a mage. It is potential that dwells within a person but does not always manifest itself. All men are connected to the Fade; we go there to dream. But only those with this potential may draw upon its power. Mana is, then, a measurement of one's ability to draw power from the Fade, and it is this power that is expended in magic.
As in all other things, it has limits. Just as a man has the strength to lift only so much weight and no more, a mage cannot work more magic at one time than his mana allows. If he wishes to work magic that would be beyond his strength, a mage must bolster his mana with lyrium. Without lyrium, it is possible for the reckless to expend their own life-force in the working of magic, and occasionally, ambitious apprentices injure or even kill themselves by over-exertion.
--From The Lectures of First Enchanter Wenselus