- Spells are prepared in advance, and once prepared a spell can be cast at anytime with little or no chance of failure, but once cast the spell is consumed; to cast a spell twice it has to be prepared twice, and so on.
- Typically the maximum number of spells that can be prepared is finite, either with an explicit cap (i.e. 'Slots'), or by requiring a finite resource (i.e. Material components).
- Variations include: more powerful spells require more slots/resources; D&D-style leveled slots; pre-requisite spells to implicitly consume extra slots for powerful spells.
- Spells consume a quantity of spell points (i.e. Mana) when cast, otherwise spellcasting is unrestricted.
- Variants are: 'Aspected Mana' (Fire mana, water, defensive, aggressive, etc., and other more unusual divisions); spells consuming things other things instead of mana (hit points, material components, food, etc.), and spells consume more then one type of thing/aspect of mana (e.g. 3 fire mana and a stick of charcoal))
- Spellcasting is essentially free, but has a chance of failure, probably a very large chance.
- A variant is to have the chance of failure be affected by some combination stats, skill, or environment.
It is quite common to combine more than one of these methods.
A Slot/Mana system might let you memorize a shortlist of spells, which are then cast as a mana system. Crawl does this. A Mana/Chance system might allow skill to reduce the mana cost of casting spells. The Pen and Paper GURPS does this.