A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.
Its basically another sun, but billions of miles away. Although stars look as though they are tiny little lights, most are actually much bigger than our own sun. They only seem tiny because they are so far away.