Different types of magic require verbal spells along with your willpower to cast. Some you can just do without thinking once you learn how.

[ Istredd pauses and considers, because that is the factual answer, but sometimes the explanation is more intangible. ]

They have spoken spells here, that's academic magic, but you still need to feel the magic within you and use it, like a muscle, like the effort that someone might use to breathe or feel or speak, but it is instinctive. It becomes clearer after the first time someone manages to cast, then they know what it feels like.