Skills are special powers which vary for each class.
The Skill-tree can be accessed via the "K" button on your keyboard.
There are active and passive skills. Active skills can be dragged from the skill tree into the skill slots in your interface. They can be used in- or out of battle. Passive skills can not be used directly but they raise various stats.
Every active skill has an invoking time, which is the time it needs to activate once used, and a re-invoking time which is the time the skill needs to "cool down" before it can be used again.
skills can be found under each class if it is done