Mana is used to play runes (cards). There is only one type of mana...a kind of "general" mana I suppose.
Each player starts off with 15 mana. Mana is then gained at a rate of 20 per round multiplied by the round # (20 x Round #).
For example, the first round you have 15 mana and the second you would have 35. But by the 3rd round it would be 75 (+40 from round 2), and by the 4th round it would be 135 (+60).
Of course this is still in the initial stages and may be changed for balance reasons as further testing is done.
Here is an image showing how mana is displayed and how it shows the mana increase from one round to the next.