The Elder Scrolls Online, like its predecessors in The Elder Scrolls series of games, features two main combat resource pools—Magicka and Stamina. Both these stats can be raised, along with Health, each time you level up. You can also raise these pools by equipping gear that confer bonuses to your stats.
To that end, every skill in the game draws from either Magicka or Stamina. At its most basic, Stamina works to supply your weapon skills (e.g. two-handed swords, bows) while Magicka is used for your class skills, such as the abilities you might use as a Nightblade or Dragon Knight.
In addition to class skills and weapon skills, the game contains armor skills, a host of additional guilds that you can join, and Alliance War skills, and more, all of which use either Stamina or Magicka.
Each of these skills scales in damage with the resource pool needed to cast it. Meaning, your Nightblade skills will go up in damage and usefulness the more Magicka you have at your disposal.
Here’s a full list of skills and their related resource pools (compiled by redditor ADevilNamedBen).
Class skills – All Magicka
Weapon skills – All stamina except restoration/destruction staff which are Magicka.
Fighters Guild – Stamina
Mages Guild – Magicka
Werewolves – Stamina
Vampires – Magicka
Undaunted – Magicka
Armor skills – Medium/Heavy Stamina, Light Magicka
Soul Magic – Magicka
Alliance War: Assault – Stamina
Alliance War: Support – Magicka
Dodge/Sprint/CC break/Sneak/Block – Stamina.
Light and Heavy weapon attacks – Bonus damage from Stamina pool.
