The Oxford Dictionary says a star is a massive ball of gas, primarily helium and hydrogen, compressed by its own gravity to the point that spontaneous nuclear reactions are triggered, causing it to emanate heat, light and other forms of energy. The sun is the closest star to the Earth. All other stars are so distant from the solar system they only appear as tiny points of light.
Stars are classified by astronomers by their spectra (measured by observed color) and average temperature. Younger, larger stars tend to be hotter, while older stars tend to be cooler. The sun is an ordinary yellow dwarf star, young relative to other stars. Betelgeuse, one of the brightest stars in the night sky, is an older red giant star. From Earth, about 6,000 stars are visible to the naked eye, but there are estimated to be over 100 billion just in the Milky Way.
The word "star" has also been applied to things that are unusual or superior; for instance, a star athlete is an athlete who excels. A movie star is a very recognizable film actor, outstanding due to beauty, talent or some other trait, and star symbols are used to call out or mark text and other items.