A spell was an ability or effect achieved through magic, and was cast by channeling planar energy either directly or indirectly. Depending on the type of spell being cast, the magic could be taken from the Prime Material or channeled via the Vivid Realms, Umbral Realms, Elemental Realms; they could also directly invoke the energy of the Positive Border, Negative Border, Astral Border, Ethereal Border.
The invocation of magic through these planes was typically referred to as "spellcraft," and one who could cast a spell of any kind was called a "spellcaster". Loosely, spells were divided into two types of casting: "arcane" spellcasters manipulated planar magic directly, while "divine" spellcasters utilized deities or primal forces to channel spells on their behalf. Creatures that had innate supernatural abilities, even those that were related to the spellwork, were not spells.
There were several types of spells, though many of their definitions changed throughout the span of time; they were often classified into Schools of Magic. Spells could also be cast by using magic items and could be copied onto Spell Scrolls or into Spellbooks, though the language of these writings was not universal and was difficult to parse from caster to caster.