Magic is a force that underlies every aspect of reality. It is able to be harnessed to bend the universe to ones whims. However, it is chaotic and can never truly be controlled. Although mortals lack the natural affinity for magic of the Immortals, they have learned to use it, with sufficient skill that they even surprise more powerful beings. They have developed various systems to organise and understand magic, bringing it in line with their limited perspective.
They have divided it into four Schools, which represent groups of related skills needed to produce certain effects. These distinctions are somewhat nebulous, as there is always overlap, such as how summoning requires both a connection to the Planescape and the ability to influence the senses of others, in order to contact beings from other planes. For the most part, however, the Schools form an intuitive set of magics, that can easily be learnt in concert.
The Schools are further broken down into Arcana. While Schools represent broad skills, an arcanum is a more specific application, such as producing and controlling fire. They are defined by core principles and techniques that can be applied to broader skills to produce specific effects.
These specific effects are called Spells. This term refers to the magical methods used to produce a specific effect, such as launching a single precise dart of fire or sending forth a fan of flame. Both of these effects are a different spell.
Of course, such an orderly system is incongruous with the nature of magic, and it is only the user's skill which keeps it in check. If the user fails to precisely modulate the parameters of their spell, chaos ensues. This is called a Magic Surge and the results are entirely unpredictable, ranging from the user turning into a sheep (inexplicably common) to the spell having the exact opposite of the intended effect to a harmless sprouting of flowers. It is worth noting that in the Feywild, a plane entirely made up of magic, these surges are both more common and more potent. You have been warned.
A curious phenomenon that is still little understood is the tendency for surges to occur during highly fateful events in totally unique and uncanny ways, that never happen during mundane spellwork.
Beyond these rules, mortal magic varies greatly. While the great magic academies tend to have very strict and precise methods, using arcane circles and words of power, less formal soruces of instructing often encourage a more instinctive approach. Each mage has a unique style, no matter how closely they adhere to there original instruction and there are countless methods that have been conceived since the dawn of the practice. In its evolution, magic is more comparable to language and culture than mathematics and physical science.