The lore of AoD is rather large to begin with, but multiple key components should be kept in mind, and these are the largest things to note.
The Primordials Edit
Creation, and destruction, the Primordials are the embodiment of the concepts that govern creation, Order, Chaos, Good, and Evil. All other forces bow before these beings, as they are the very source of Quintessence, the fabric of existence. Their actions govern much of the world's events, even if they are never seen directly, their influence is everywhere. The greatest antagonist of the series, Apothis, is one such Primordial, the creator of the entire multiverse, and the one who now seeks its destruction. Only by quelling Apothis' fury may the Universes find peace.
Quintessence and Magic Edit
Quintessence in short is the smallest form of essence one can reach, atoms and energy, every single thing that exists is composed of Quintessence. To not have Quintessence is to not exist at all. This fundamental force is the source of Magic and Alchemy, the manipulation of supernatural energy and the manipulation of the physical laws of the universe. Quintessence dictates how much power something has, and even determines the forms living beings may take, the more power, the higher in the evolutionary tree they climb, the less, the lower.
Those who use magic, traditionally called mages by some, but in canon are generally called Awakeners, fall into multiple categories and types, being the True Awakener or Black Mage, and Shamans or White Mages.
True Awakeners use the power of the Primal Realm, taking vast stores of energy, and give up nothing in return. These mages are destructive and battle oriented, a result of those who seek power, not harmony. They hold vast potential, able to weaken and make feeble their enemies, smiting them with fire from the heavens soon after. These Awakeners are dangerous however, not only to their enemies but themselves or even the world around them, as Awakeners possess a deep connection to the Primal Realm, and when used improperly, may result in them transforming into a Primal Gate or even a Elemental Primal, in either event, a catastrophic chain reaction soon occurs, known as a Primal Cascade. This Cascade results in the transformation of the entire world or plane into the element unleashed from the Primal, resulting in the deaths of all who inhabit the world. For this reason Awakeners are observed with great caution by Deities, who make sure that these mortals do not grow too hungry for power, or are used for malicious purposes.
Shamans on the other hand, focus on balance and healing, while capable of offense, are more concerned with the healing, protection, and empowering of those around them, giving and taking in equal measure from the Primal Plane. They are often deeply in tune with nature, and so less civilized nations adopt them as their form of mage, while well established ones turn to Awakeners.
Clerics and Paladins should not be confused with the Awakeners and Shamans, as their power is less magic and more divine power, drawn from a particular Deity of their choice, wholly dependent on their patron for power, as opposed to mages, who rely on themselves.