Mana is a side effect of Life. It is a primordial material (similar to Arcane energy). Mana differs from Arcane energy in that it is potentially anything. How this energy is manifested depends entirely upon the living creature focusing it. For example, it can be focused into such diverse things as Food, Lightning, Protective Barriers, and Space-Time Manipulation. The general designation for the focusing of mana is a spell (e.g. Raise Dead). Mana has become a cornerstone of societies the world over, and is even routinely used by angelic beings and demons.
All living creatures, rats, Elves, Orcs, and trees, contain mana within them. Even corpses contain residual traces of the mana they once produced. Mana can also be produced by runic cultures on bones. It is theorized that mana is produced at the cellular level, akin to ATP, by producing agents called "mana-chlorians".
Mana is measured by the clever use of "mana rings", devices which display a "mana-meter" of sorts. Mana-meter is itself a spell which uses a small amount of mana borrowed from its host to measure mana. They can be identified by a blueish band of pure mana (or in the case of amulets, a blueish pattern). These meters do not use numerical ratings but instead give a basic indication of the amount of mana stored, the rate of mana reproduction, and the fractal amount of mana used in spells. Whenever mana is taken from the pool the band will deminish by a fraction, but it will be regained overtime either by natural production or the consumption of distilled mana in a potion. The more mana contained in a being, the brighter the band of mana.
For the vast majority of sentient beings, mana-meters produce extremely faint bands. Those unfortunate people will find learning spells to be nigh impossible. It is rumored that the mana challenged among us could possibly suddenly develop their mana-chlorians. Whether or not your children will be able to use mana is thought to be genetically rooted. However, there are several documented instances of atavisms. Atavisms occur when the ability to use mana suddenly appears in a gene pool thought to not possess the talent. This is thought to be proof of a common lineage which binds the Heroes to the commoners with one common ancestor, Eve.