What is a supernova?
A "supernova" is a nova (Latin "new") explosion of a very large star, creating a nebula of illuminated gas that is one of the most visible of all celestial phenomena.
Remember that a star is a nuclear fusion "engine" that is powered by the fusion of lighter elements to create heavier ones (up through iron). As lighter elements are fused to create heavier ones, energy is released in huge quantities. Gravity holds the star together against the outward force of all the fusion reactions, and the star operates in equilibrium. The extreme energy in the star allows for fusion reactions to continue to occur. But eventually the core of the star is largely sulfur and silicon. The contraction and heating of the star's core allows it to begin massively fusing silicon into iron, and this reaction absorbs energy rather than releasing it.
At some point, the energy created in fusion is insufficient to hold the star "up" against its own gravity, and, having exhausted its "regular" fuel, it will collapse. This is the supernova process. The star collapses causing the outer layers to violently explode outward as gases and dust. The compression of the body of the star in the collapse creates enormous thermal energy that supports the endothermic fusion reactions that create those trans-iron elements (up through uranium). It all occurs in a relatively short period of time. Afterwards, the remnant core may become a super-compressed neutron star, or even a black hole, if the star is massive enough.
Supernovae help enrich the interstellar medium with heavy elements, and shockwaves from the explosion may also trigger star formation. It is believed that the Sun and Solar System formed as a result of a nearby supernova.
While a supernova has not been observed from Earth since 1604, supernova remnants indicate that they occur, on average, about once every 50 years.
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What effect may a supernova have in a star?
A supernova will usually spell the death of the star. Much of the star's material will be blown into space. In the case of the important sort of supernova called a Type II supernova, what remains will become a neutron star or a black hole (depending on how much mass is left after the supernova explosion).