They all start out as level 0, then you decide which ones you want to use. The higher the level, the better the skill. Spells usually last for only a short time. They rarely have a sustained effect. They usually are used for offensive combat.
Spells require mana.
Auras have a sustained effect. When an aura is activated it will usually effect everything within a given radius around the player. Auras are usually used for defense, they stay activated until the user has no more mana, or has disabled it. Combat skills . Comba skills are used in weapon combat.