To those Weavers who are interested in philosophy and theology, there are plenty of debates on the origin of the Arcane. The most popular (and the one the writer - me - deems true enough to use it in the story :] ) is the one of the Spheres. There are seven spheres in total: Earth, Air, Fire, Water, Life, Decay & Chaos. Each of them has been appointed to an Archon, a 'divine' of sorts. Whether the Archon is the sphere (and thusly allows Weavers to tap into its own lifeforce), sources the sphere (it is to say, created it and offered it to use) or guards the sphere (you never know when it explodes and mayham ensues?) is another heated debate of Weavers.

The first six spheres are known to the World, and is used plenty for any magic craft. The seventh sphere is, you'll guess it, another heated debate of the Weavers. In any case, the Sphere of Chaos is appointed to Wyrd, the Archon of the Netherworld, and is therefore associated with the energies of the dead. Living things have no access to this sphere, logically, but philosophers may tell you otherwise.

The spheres allow Weavers to create magical effects under the following sort:

Earth - Sand & Primal: to be understood any effect that creates or manipulates the very soil under your feet. Sandstorms, earthen walls, pebbles & boulders... Do mind that movement of said pebbles is not Earth magic, so an Earth Weaver can't push a resting boulder into motion with this magic.
As for primal influence, we refer to basic instinct and innate abilities of nature. This could go as far as a heightened regeneration of one's body or health, feral allure or the enhancing of one's voice, manipulation of anger into frenzying (or otherwise).
Air - Lightning & Force: UNLIMITED PO- erm, wrong story. The sphere of air rules over movement and energy. With lightning is to be understood the overwhelming bursts of energy. This could manipulate the amount of fatigue of people, or (mostly) downright shock them dead.
Under the category 'force' falls telekinesis in many forms. The movement of items or people at your whims, the altering of speed, the impact of gravity upon a body or plainly the summoning of the 'invisible force' known as wind.
Fire - Heat & Acid: Fire leaves little to the imagination, to be honest. Heat burns. Simple. May it come in the shape of fireballs, supphocating warmth or retaliating auras, heat's purpose is to prevent cold, at its nicest point, or to boil and burn things, at its typical point.
As for acid... it's not much different. Acid burns through just about everything it touches. It's mostly used to destroy equipment and items, but... not all Weavers are careful, I guess.
Water - Cold & Poison: in direct opposite to Fire, Water will cool what's heated and douse burning flames. But don't misjudge. When the cold drops to below zero and more, we're not talking friendly airco Weavers anymore. Slowing movement to a halt, creating slippery floor, turning people into living icecones... worse has been done. In a different direction, Water provides means for scrying. That is, the means to look into 'different worlds', thusly altering the sight of people in the World.
With poison... one should think about blood. Blood is a "Body's water", and what better to use liquids and venoms of all sort to manipulate the actions of someone else? Morbid? Certainly, but the touch of death has rarely been associated with warmth, hm?
Life - Creation & Radiance: not all spheres are out to destroy though, don't worry. The life sphere might probably be one of the less offensive spheres known. Creation might be easiest to define as a form of conjuration. It allows a Weaver to create 'materials out of thin air'. However, considering the nature of magic, those items will never remain. Sooner or later they will disappear. The other use is a most benevolent one: the 'creation' of lost health. Healers of all sorts usually rely on this sphere to enhance their healing abilities, even when using herbs and balms.
With radiance, one must think about light, the light of auras and the light of memories. History has been written through actions, but the 'radiance' of the people who defined it is what is remembered as a good or bad influence. Radiance is the light of the sun blinding the fools staring too long at it, and the light of truth piercing the darkness - it's a great power to destroy illusions and undead alike.
Decay - Necrotic & Darkness: where there's light, there will be shadows. And of course, if the sphere of Life crumbles illusions and undead alike, then Decay will bolster their existence. When one thinks of necromancy, one thinks of undead puppets and vampirism. And it's exactly that, too. Necrotic is the pure power of decay, making anything crumble that stands in front of it.
Darkness compliments that power just fine. While it is more a power of the mind, invading happy thoughts with depressions and phobias, twisting dreams to nightmares, turning day to night and light to darkness, this power is one shrouded in mysteries.
Chaos: This sphere isn't part of the Arcane. It's part of the Nether. Therefore, living creatures can't weave Chaos (also known as Nethermagic). The sphere of Chaos holds all possibilities of the previous spheres, but where the Arcane taps into latent energies, Chaos taps into the very essence that creates an individual.