What is the difference between a supernova and a red giant?
Both supernovas and red giants are dying stars. Medium sized stars become red giants, and very large stars become supernovas.
Both supernovas and red giants are names for a stage in a star's life – the stage when a star is dying.
Very large stars (8-10x the size of the Sun) will explode into a supernova when they die. The explosion is so large that the light from the star will outshine all other stars in the galaxy.
Medium sized stars (like our Sun) will turn into a red giant as they die, and eventually become white dwarfs.