Magic is one of the key components of the world of Tylien, the cause for the rapid advancements of the Giants, and one of the gifts passed down to mortals from the gods. Most magic is derived directly from reaching out to an Ancient God (or a corpse of one) and drawing on them for power. The type of spell is based on the Ancient God (or corpse) that you draw upon. You'd get fire magic if you drew from Pyrofaltic, for example, and ice magic if you drew from Cyrotix. The precise spell is drawn from what pattern you draw in your mind. Think of it this way. The Ancient God is a physical point in space, and the magic that comes from it is a physical beam of energy. Magicians usually focus on shaping the beam of energy into a particular way, so that when the beam makes it's way from the God Point and hits them, they channel the energy into a spell.
Casting magic though, is extremely dangerous. It starts out with only a slight risk of danger for beginner spells with small amounts of channeled energy, but with higher level spells and higher amounts of energy that require much more magic to cast, the risk grows more and more. It is imperative for higher level mages to have complete focus and mastery over the spells they know, seeing as messing up major spells has dire consequences, opposed to the slight injuries or headaches that might occur from botching a minor spell, like the implosion of the brain, or paralysis. This is why it's so rare to see mages of a higher level, as most magicians don't actually have any interests in warfare or battle, most being farmers or artisans.
To make this process easier for novice and intermediate spellcasters, some use verbal incantations and movements to better visualize the spell and to better focus on the task of casting magic. Most novices do both, intermediates only do movements, and masters to neither, only using mental commands. For battle mages, it's simply unacceptable to say out loud what you're about to cast, as it gives away crucial information about your coming attacks to the enemy. Most movements are archaic to the average infantryman, but going against those trained in the art of magic, movements give away just as much information as shouting out the spell does.
The process of creating or discovering new spells is a perilous one. To discover a spell, you basically have to shoot around in the dark with energy combinations and hope to find a new spell. This is very deadly, as a singular wrong move could potentially end in a fatal disaster. Because of this, advancements on new spells has remained mostly stagnant, as very few find themselves willing to actually sacrifice themselves in the pursuit of the advancement of magic.
TLDR: Magic is based on the will of the caster, who draws on the powers of the gods to channel the will into tangible magic.