Why is the structure of the sun different from the structure of earth?

The difference between the structure of the sun and earth is that in the middle of the sun is the core. The core is where ENERGY is produced...The ENERGY is transported to the sun's surface through the RADIATIVE and CONVECTIVE zones.

(Coming from a 6th grader)....

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That gives a rough idea of how the Sun works, but a more useful answer to the actual question is that the Sun is a star - a sphere of extremely hot hydrogen gas undergoing nuclear fusion to helium; while the Earth is a planet comprised mostly of rock (though its Core is believed to be the metals iron and nickel).