The Water Margin
On the dying world of Athas, all resources are hobbled and crumbling. Magic is no exception. The mystic arts carry significant limitations, simply because even magical power is given up only grudgingly by the exhausted world.
All arcane magic on Athas stems from life-force. As the planet is widely denuded, such energy is precious and hard to come by.
Wizards draw their arcane energy from living creatures. Most mortal magic will kill plants and cause pain in people; extremely powerful magic can actually injure or kill animals and people as well. A defiler draws out such energy recklessly, with commensurate destructive effects upon the environment. A preserver attempts to use energy sparingly, in order to prevent further damage to the world. See defiling and preserving for a more detailed examination.
Since Athas has no gods, divine magic is the province of petty would-be deities and distantly anthropomorphic entities.
Clerics serve elemental lords. These lords inhabit the elemental planes that have tenuous connections with Athas. Because the elements jealously guard their provinces and constantly struggle against one another, each cleric is devoted to a particular element and may only draw upon spells tied to that element. The elements also demand that their priests use weapons, armor, and tools appropriate to their calling. The elemental lords are often uncaring and distant from humanoid creatures, and their motives can be inscrutable, so pursuing the path of the cleric requires single-minded devotion — even then, the elementals may, at a whim, refuse the obeisance of a would-be acolyte and bring only death.
Druids watch over and protect specific features of the land. A druid oversees the animals, plants, and natural forces of a particularly unique or majestic place. The spirit of the land itself communes with the druid and provides spells based upon its own form, so a druid watching over an area of swampy mud flats will gather different magics than a druid who protects a ridge of volcanic fissures. The spirits of the land require druids as their intermediaries with the mundane world; while the spirits can grant magical gifts, they require the aid of druids in protecting their places of power from intruders.
Templars gain divine power from the sorcerer-kings. Each sorcerer-king possesses the ability to gift servants with divine spells. A templar gains these spells through reverence or meditation. Unlike clerics and druids, templars are not beholden to an elemental patron or lord, but the sorcerer-kings guard their power closely and thus templars will find that the acquisition of knowledge is a greedy, grasping process. Of course, templars are also expected to serve in the city bureaucracy, and as such they msut bow and scrape to their superiors, perform various mundane tasks, and participate in the dangerous politics of the city-state. The sorcerer-kings are not true gods and are limited in the power that they can distribute (although this fact is not widely advertised), and as such there is a definite upper limit to the magical prowess of a templar.