A super giant star is the exact same thing as a giant star only much bigger. Remember that as a star gets older it begins to run out of fuel. As the star runs out of fuel, it will start to burn out.
Just like the sun-sized stars, blue giant stars also begin to burn helium. As they do these stars get much hotter. This extra heat makes the outside of an old blue giant star stretch out further. Remember how hot air balloons stretch out as the air in them gets hotter?
The only difference between Giant Stars and Super Giant Stars is their size. Super Giant Stars are much bigger. If the Sun were replaced by a super giant star, it would extend from the center of our Solar System almost all the way out to Uranus.
Did you know?
Dead super giant stars/blue giant stars often turn into black holes. A black hole is a very compact object. Learn more about black holes. How does this happen? As the star dies it explodes in a huge explosion called a supernova. The supernova blasts away most of the star. Anything left behind begins to fall into the middle of the star. It gets more and more compact, and smaller and smaller. If there is enough of the star left after the explosion, the star will be heavy enough to squash it down to the size of an atom, or even smaller.