Is the sun really fire or is it something else?

No. The sun is not really made of fire. It is made of plasma; gas so hot that electrons start to break free of atoms. The process that powers the sun is not burning as we understand it, but nuclear fusion. Burning, or combustion as scientists call it, involves some flammable substance undergoing a chemical reaction with oxygen. In such a chemical change, atoms rearrange how they are bonded to one another, but the atoms themselves do not change.

At the center of the sun, hydrogen atoms fuse together to form helium atoms. The nuclear fusion in the sun is releases millions of times for energy than you would get from burning the same amount of fuel and produces far higher temperatures.