Magic in Dark Sun like magic in D&D has two major derivations Wizards magic (aka arcane magic) and Priestly magic (aka divine magic). Psionics are not magic. Psionics are natural psychic phenomenon
Wizard Magic in Dark Sun requires the absorption of energy from something and redirecting it to power spells. Most wizard magic draws from life energy though there is at least one Sun Wizard in canon. Wizards that draw their magic from living things come in two flavors: preservers and defilers. Preservers draw the life energy out of plants very slowly and in limited amounts so as not to kill their power sources. Defilers draw as much life energy out of plants and animals as they can and as quickly as they can. Defilers leave large areas of destruction in their wake as their process of absorbing magic energy kills all life and destroys all soil in a radius around them each time they absorb magic energy (usually every morning). The box set provides information on how large a defilers radius of destruction will be. Higher level defilers absorb more energy and thus destroy more land and life to power their magic.
The sorcerer-kings are the penultimate defilers. They do not tolerate any other wizards in their domains as the presence of to many defilers in an area would risk their city’s verdant belt and preservers have an ideology that makes them natural enemies of most defilers. That said most sorcerer-kings make an exception for few pet defilers they keep to perform various tasks the sorcerer-kings consider beneath themselves. These pet defilers can use their sorcerer-king’s trees of life to draw magic energy. There is usually at least one pet defiler that works in each city’s arena performing various magic tasks needed for the operation of the arena.
Priestly magic in Dark Sun is all derived from the inner planes, and is tied to one of more elements. Clerics gain their magic by forming a pact with beings on an elemental or quasi-elemental plan. Druids form pacts with elemental spirits to gain their magic from the elemental planes. Templars gain their elemental magic from conduits that were opened from the elemental planes thousands of years ago with the creation of the sorcerer-kings. Each sorcerer-king controls access to these conduits. Sorcerer-kings release magic from these conduits to their templars at the telepathic request of each Templar. This is done in a ritual fashion that does not take conscious thought from the sorcerer-kings. Though each sorcerer-king has the power to block any of his or her templars from accessing these conduits. Unfortunately sorcerer-kings cannot access these elemental conduits directly for themselves, but they can grant or deny access to these conduits for thousands of their templars.