Mana is the magical energy generated and stored in all living beings. It is required to cast spells which may be trained or, in case of some creatures, used instinctively. As the aptitude for magic tends to scale with intelligence of the species, so does the maximum amount of mana an individual can carry, the ‘pool of mana’.
Of all species humans have most variation in both the mana pool and the rate which they regenerate it. Like mental and physical abilities, mana is tied to both inheritance and practice. Daily training of spells will increase the mana pool, just as regular exercise will increase lung capacity. Talented individuals may see their ability grow faster than their peers.
Spells are not completely harmless to the caster. Heavy mana usage will result in weariness and complete depletion of the mana pool may knock the caster unconscious. The caster’s abilities determine what counts as heavy mana usage. Seasoned mages are hardly affected by a spell that could deplete the whole mana pool of an inexperienced caster.
Mana regenerates all the time at a pace which is affected by numerous factors. Hunger and weariness greatly decrease the regeneration, which is why most mages sleep long and never miss a meal. Physical fitness is beneficial, but often neglected.
Mana can be transferred to another person, an application of magic often used to aid healers in difficult medical operations. Draining mana from an unwilling subject is also possible, but highly illegal in most countries. There is a lot of grey area between these two, as mana is highly sought after and traded with all kinds of agreements.