* Stars are giant balls of hot gas that produce their own light and heat through nuclear fusion. The Sun fits this definition perfectly.
* The Sun is just one of billions of stars in our galaxy, the Milky Way. It's a fairly average star in terms of size and brightness.
So, the Sun is a star, not a "sun star". "Sun" is the name of our specific star, while "star" is the general term for any celestial body that fits the definition above.
