[quote=Scarifar]
There's energy we use to activate skills? I assumed all that hindered skills was cool down time.
[/quote]

Might be the case with some. Typically the way I think of it is like this:

Melee Skills require "Adrenaline" meaning that they need to build up energy during the battle. Some players might be able to access an burst of Adrenaline right off the bat, allowing them to use certain skills immediately. Items and skills can be used to increase the rate Adrenaline is made or your initial amount. Adrenaline could also be mana, but the method is the same regardless.

Mages uses mana. Pretty simple; you have a set limit and you must conserve how much you use before you're all tapped out. Restores over time, through items, or with certain skills/abilities.

Technical skills like those using guns or certain power modes require "Cooldowns", being that you can use them whenever you're able to, but afterwards they need time to recharge before you can use them again. Probably items to decrease cooldown cost.