Someone who's joyful is very happy. A joyful child will laugh with delight.
When you experience the feeling of joy, you're joyful. For many people, their wedding day, the birth of their children, or simply a beautiful summer afternoon can all be joyful occasions. To be joyful is beyond being happy — there is delight and a kind of bliss involved in feeling joyful. The noun joy is at the root of joyful, from the Old French joie with its Latin root of gaudere, "rejoice."