Magic comes in an infinite variety of forms. A gifted few are able to channel internal energy to create spectacular, if ephemeral, effects. Those with fey blood are particularly adept at this, often being born with a talent they can use as intuitively as you or I would move our hand. The gods may also choose champions from among the mortals, and the faith of the champion can create miracles. Even among those born without such gifts, the secrets of magic can be unlocked with long work, but harnessing such power is difficult and time-consuming.
Psions, as those who can express their own power are generally termed, typically have a very focused ability. There is one particular trick that is at the core of the various effects they can produce. For some, it may be moving objects with their mind, for others a knack with illusion, and for yet others an affinity with a particular element. Their magic typically comes at the cost of fatigue.
Those chosen by the gods are also very focused, able to manipulate the forces that answer to their deity. Unlike psions, though, their magic is not powered by their body, but by their piety. They must observe rituals, taboos, and commands set forth by their religion. These restrictions are often much more severe for the chosen than for the ordinary worshiper. As they perform acts which please their gods, they are rewarded with power. Should they perform an act which displeases, their power may be stripped from them as a lesson.
Ritualists have the advantage of being able to produce any effect they like, so long as they can discover the correct ritual. However, the rituals are difficult, results are hardly guaranteed, and each ritual takes a great deal of time to prepare and cast. It is a well-established fact that the only way to subdue a wizard is by taking him by surprise.
Precisely what magic is, where it comes from, and why it obeys such peculiar laws has been the subject of much speculation through the ages. There are a few general principles that even the layman can appreciate.
Magic is, in simplest terms, the energy of life itself. It is what allows a living creature to move and grow, where a dead creature does not. As each creature lives, it converts food into magic. As each creature dies, some of that magic is left behind in the world. Most of this energy, of course, is consumed in the simple act of living. But enough traces are left on every object that you touch to eventually create a vast, invisible sea of power around us.
The question of why we have magic at all, or how we know how to manipulate it, is a thorny one. Primarily because every religion, from the most venerable institution to the most obscure cult, attempts to explain it with their own mythology. The common explanation is quite simple. Magic is one of the Ten Elements that make up existence, and exists for the same reason that the air we breathe and water we drink does. We can manipulate it because some higher being, most commonly one of the gods, taught our ancestors how.