Casting Requirements

Spellcasting can be a complicated process requiring intense concentration while completing complex movements along with precise incantations; some spells may even require strange arcane reagents as a focus.
Other times it is as simple as weaving together the innate magics of the planes to create an effect through the patterns of music or force of will, whether that be the caster's own will or the will of greater entities channeling through their vessel.
Casting spells draws partially on the caster's innate willpower, or the will of some other planar being. Students of magic will learn quickly that their minds and bodies can only draw upon so much reserves of magic before they will need to rest.
Magic is considered a very dangerous power to wield, because of this it is often a closely guarded secret. Those that seek to learn powers from others or draw forth innate magical abilities from gifts or magical items are often considered "dirty" or "unclean" magic by those that learned the complex theory and correct components required to master spells.

Somatic

A basic part of spellcasting, somatic components refer the specific movements of the caster which require them to be unhindered so as to allow them to complete the motions smoothly and safely. Not doing so might result in the spell not working at all, or creating some other unintended result.
These movements are believed to be the "will" part of magic, the concentration and the movements focus the mind, often with mantras or repetitive sayings that guide thoughts as they invoke the powers of magic.

Verbal

Another important part of magic, verbal components to a spell are those words that need to be spoken to give the magic shape. If the speaker is interrupted or silenced before they can complete the words, the magic can fail as its power is drawn forth but it its not expelled.
The "word" part of spellcasting is often in the language of the arcane itself, a scholarly language that is almost never spoke aloud except in instructions. Such words are hard to remember, almost having lives of their own. Many are eager
to return to the magic from which they were spawned and be freed from the prison of the caster's mind. Binding the arcane symbols to the written word requires enchanted inks and parchment, as mundane inks and paper seldom hold onto the symbols and runes long enough. An untrained reader usually finds the language indecipherable nonsense.

Material

Many physical objects in the world have power inherent to them, and magic can be focused through them to produce fantastic results. Many wizards and arcanists employ innately magically objects such as rods or wands to focus their spells without relying on a combination of magical components that needs to be gathered and maintained. Some items however are required for certain spells, such as a mirror or crystal ball to allow the viewer to cast their mind's eye elsewhere, or a pearl's ability to allow a caster to identify magical effects and powers.