Mana is a form of energy that is used for using abilities. There are 6 types of mana are fire, earth, air, water, light and shadow. It exists within people's bodies, and it flows through the world similar to wind/water currents. For example, you can find high concentrations of fire mana near volcanoes, lava lakes, etc.
The higher level someone is, the more infused their body would be with a type of mana (In noob speak, the higher the level the more the mana), the type likely depending on the race and class of that person. Having a higher level would also cause the mana to regenerate faster. The less mana one has, they more tired they become, and once they hit zero or less mana, they collapse (OOM). There is one mana bar, which shows how much mana a person has left, though a person would likely be able to "feel" how much mana they have left without it.
How the types of mana and the amount of mana relate is so far uncertain.
Mana is a requirement to use abilities. If you attempt to use an ability without enough mana, you will collapse (whether the ability is successfully casted before you collapse is still uncertain). How "Channel" abilities work is still uncertain. Passive abilities do not consume mana.
How the types of mana and mana consumption relates is so far uncertain.
Maximum mana is affected by the Intelligence stat. The higher your Intelligence, the more mana you can have. Devotion affects mana regeneration, with a higher Devotion increasing the speed with which your mana regenerates.