What is the difference between the sun and a planet?
A star is a big ball of gas (hidrogen mainly) undergoing the process of nuclear Fusion and, in doing so, producing energy.
A planet is a ball of inert material (silica for example) that receives light and heat from a star.
Due to its mass it is almost certain that the Sun will always have planets revolving in orbits around it while the opposite is highly unlikely.
The Sun also will undergo changes in its structure due to the depletion of the components forming it that are reacting in thermonuclear fusion. The Sun will consume its hydrogen changing it into helium and continue to burn until finally it will "die" changing into an inert, dense and cold body.
A sun is a star it is significantly larger than a planet and emits energy due to fusion reactions.
A sun or star is a large body consisting of mainly Hydrogen and helium. It is massive enough for fusion reactions to take place in its core. The fusion reactions generate a lot of energy which make a star shine.
A planet is any body which orbits a star and has cleared its orbit of other bodies. A planet can be anything from a small rocky body to a large gas giant.
To put it in perspective the Sun contains over 99.8% of the mass of the solar system.
Some people think Jupiter is a failed star. It is not. It would have to be over 80 times its mass to start any form of fusion reaction.