What is the difference between a red giant star and our sun?
Size and age. As many stars age, they swell up to Red Giant stars in old age and become huge.
Stars progressively burn up their hydrogen fuel as they age and towards the end of their existence they swell up to become Red Giant stars. Average size stars become Red Giants, collapse and then become white dwarf stars (upper pathway in pic).
Massive stars also become Red Supergiant stars and then go supernova and then either become neutron stars or black holes.
Our own sun is about middle aged at 4.5 billion years and has another 5 billion years before it becomes a Red Giant. Once it becomes a Red Giant, the Earth will likely become absorbed by the expanding star.