Classic Mode ( Normal Mode) is an option that can be selected upon creation of new Worlds / the only available World mode. As its name suggests, Classic Mode worlds offer Terraria's "normal" gameplay experience which existed prior to the introduction of other world modes. As such, Classic Mode worlds do not feature the "creator" freedom of Journey Mode worlds, but nor are they as challenging as Expert Mode or Master Mode worlds.
While the world content of Classic Mode worlds in general mirrors that of the other worlds modes, it does not hold any Expert-exclusive items, Master-exclusive items items, nor the Fledgling Wings. There is no Classic Mode exclusive content.