Mana is a form of energy that humans (mostly Templar) and Keidran can use for their magic.
Dark/Black Mana
Templar can draw Mana from the ground, resulting in a large supply of energy, but if it runs out, they will start using the life force of the planet. This is also called Black Mana, and though it is much stronger and less resistable than normal mana, it can corrupt the user and destroy their minds if they are exposed to its effects for long enough.
User Species
Keidran
Keidran (and non-templar humans) are unable to draw mana directly from the ground and so need to use crystals containing pure mana. These provide a small supply of mana, a new crystal is usually required for each spell, however they can recharge over time. In order to utilise the mana stored within one of these crystals the user must be making direct contact with it, meaning that the user must be holding the crystal (or otherwise touching it, for example as a necklace) to cast a spell. Like Templar, crystal-users can make use of black mana which can be obtained from "the crystals formed of the dying earth," although they cannot use it accidentally like the Templar can.
Basitin
Basitin are completely unable to draw their own sources of mana (from crystals or directly) and so are unable to use magic. Basitin can, however, draw from a large reserve of mana within a tower, but with terrible consequences. Because Basitin cannot naturally react with mana, their minds cannot handle its effects. When Basitin are connected to such a huge reserve as a tower for too long their mind gets corrupted as it slowly snaps under the strain of Mana. By doing this, they can use magic for a limited amount of time. After being exposed for too long, however, their mind stops functioning properly, and they lose the ability to use magic, and they cannot even form full sentences anymore.
Uses
Mana can be controlled by a mage (of any race) to manipulate the physical world and everything in it. The series has shown many exact examples including: conjuring, attacking, defending, teleportation, manipulating elements, controlling people, and healing; however it can be assumed that the manipulating effects of mana are almost limitless.