This costs a small amount of Mana points(MP), depending on the level of the skill and the skill itself.
They do more damage if the level of the skill is higher, but some skills are buffs, so instead of attacking a mob they increase your stats, depending on thier level.
Some skills are passive, they can't be activated. They usually give you some kind of a boost, like movement speed or damage.
Their effect is active as soon as they reach level 1 and from there increase thier functionallity for every level they get.
The passive skills are in the skill window orange (see picture to the right), While the active is green.
The grey ones are the ones that aren't useable because they are level 0.