A spellcaster may refer either to a character who can use magic, or to a creature capable of casting magic spells.
A spellcaster character possesses at least one level in the Spellcasting skill, and may be proficient at any other number of magical schools, such as Conjurations, Translocations, etc. Spellcasters use MP to cast spells they have memorized from spell books. Characters can begin the game with a spellcaster background (e.g., Wizard) or they may opt to train Spellcasting as they gain experience.
A spellcaster monster has access to a limited set of spells they can cast. Monsters do not have MP and may cast their spells as often as they like. Spellcaster monsters tend to be dangerous but frail. The power of a spellcaster monster's spells is determined by their hit dice. See the spellcaster category for a list.