Gandalf is a Maia in J.R.R. Tolkien’s legendarium.
The Maiar are supernatural spirits created before the physical world was formed. They are lesser than the Valar, the most powerful guardians and rulers of the world, but they remain far beyond ordinary mortal beings. Gandalf originally belonged to this spiritual order under the name Olórin.
He was sent to Middle-earth in the Third Age as one of the Istari, or Wizards. In that role he appeared as an elderly man and was expected to guide the peoples resisting Sauron rather than defeat evil through overwhelming power.
Readers sometimes describe Gandalf simply as a wizard, which is correct for his role in Middle-earth but not his underlying nature. His transformation after confronting the Balrog reflects his status and mission, not a change from human to supernatural being.