Mana is the life-energy of any living being. It exists within all living things, but only a fraction can control it, and an even smaller fraction possess enough mana to properly cast spells. Mana is in constant circulation around the world and often moves as if by itself; condensing strongest where it is needed or where magical energies are most likely to thrive. It is an energy that is essential to the world's existence and balance, and it is more or less infinite. When mana dissipates or is formed into pure magical energies, it will slowly shift to somewhere else in the world.
When someone adept at magic casts a spell, he or she draws mana from their inner mana pool to trade it for magical energy. It is this magical energy that shapes and fills the cast spell. The more spells a person casts, the less mana he or she will have in reserve. If the individual was to completely drain his or her mana reserve, he or she would suffer potentially fatal drawbacks.
Mana is replenished through rest and nourishment. Some potions also replenish mana at a rapid rate, though over-consumption can lead to addiction and irreparable damages to one's mind and body. Thus the consumption of mana potions are to be taken with care and consideration.
A mage will, throughout his or her life, slowly expand the available pool of mana of their body. Most spellcasting individuals will be born with an average sized mana pool, and every individual has a different size of mana available. Some have larger quantities than others, though it is often quite miniscule in difference. But the general rule is; the more one understands about magic and the more one meditates throughout the years, the more mana one can draw from.