Mana is an energy which is vital to casting magic, it is a form of energy which makes itself home upon the dimension of which souls reside in. A powerful magi can rarely cast continously for longer than ten minutes without taking an extended break.
Mana can be created from the soul, and is limitless in the dimension of which the soul inhabits. The limiting factor to how much mana an organism can call with their soul to their mana circuit, and how much of it one can control within it at once.
A central property to how mana can be used in our physical realm is how unstable it is when not controlled by a governing soul's willpower. When left untampered within our dimension, it will transmute itself into whatever energy there is readily available, ergo, heat, pressure, radiation, and perhaps even mass itself (if there is enough energy).
Its colour, appearance, and lustre is defined by the soul's affinity, and what the soul's inhabitant perception of the affinity to its respective colour.
When a person controls mana, they, through their commands, contact their soul-- which is an almost instantaneous connection because of their mana circuit-- and their soul manipulates the mana of which has been sent into the physical realm by its control.Energy Magic), and is usually done so lethally as in its pure form, it may be a substance which can be extremely detrimental to anything it makes contact with.
Mana as a Sense
Many Magi grow familiar with the energies they use, and some whom have mastered their manipulation of mana can grow to use such an energy as a "sixth sense" sort of technique of which they can send certain energies out magically and gain feedback from it. This is how magicians can actively control their spells such as Sear (see Fire Magic) or Meteor (see Cosmic Magic).