A star is a large luminous celestial body made out of matter in the plasma state, held together by the gravitational force.
A star is formed when a large cloud of interstellar gas condenses into a massive round body that generates internal energy by means of nuclear fusion, fusing hydrogen atoms into helium atoms. During the course of its life, the star emits energy, electromagnetic radiation (light, microwaves, gamma rays and more), neutrinos and other particles that make up solar wind. Thanks to their light, thousands of stars are visible to the naked eye in the night sky, regardless of the fact that they are many light-years away in distance.
Stars exist in many possible sizes, ranging from small long-living red dwarves to enormous blue giants. As you can see, the color of a star is strongly related to its size. Stars are divided in seven classes: O, B, A, F, G, K and M. O-type stars are the largest ones and are usually light blue in color, whereas M-type stars are the smallest ones and are usually red. Average stars, like Sol, which is a G-type star, are yellow or white in color. But regardless of being average in size, these are not the most common kind. The most common and most long-lived stars are the red-dwarves. The larger the star is the less common and more short-lived it gets. That's because although large stars have much more hydrogen to burn, their metabolism is must faster and they generate energy at a much faster rate.
When a star runs out of hydrogen, it starts to fuse helium into even larger elements, such as carbon. During this brief period, the star grows to gigantic size and becomes red in color, thus being known as a red giant. After this, the star explodes, generating a cloud of gas containing many kinds of substances generated during the course of its life. Most of the heavy atoms in the universe were created inside stars and released in these huge explosions called supernovas. The star's remains form a very small and dense body known as a white dwarf. Depending on the original size of the star, these remains can condense into even denser bodies like neutron stars or even black holes. After countless years, even white dwarves and neutron stars will stop shining and the star finally dies. Hopefully, the materials released by the supernova explosion will eventually condense into another star and the cycle of life begins again.
There is, however, a minimum mass a gas cloud must have in order to condense into a star. If it fails to reach the minimum mass, nuclear fusion will not take place and the object formed will not be considered a star, but rather a brown dwarf. Brown dwarves do not generate energy and are very similar to gas giant planets, only larger. Brown dwarves may even be found orbiting around regular stars. Thanks to their large size, even small stars usually have many objects in orbit around them, such as planets (and their moons), dwarf planets, asteroids (often forming asteroid belts), comets and artificial bodies like space stations. Together, the star and all objects gravitationally bound to it make up the star's system. A great number of systems actually have two or more stars, gravitationally bound to each other. These are called binary or trinary systems. Usually, one of the stars is larger than its companion. In a binary system, planets may be found orbiting only one of the stars or the other; or orbiting both simultaneously, in case the stars are very close to each other or the planet is too far away from both. In some regions of space, the concentration of stars is much higher than normal. This happens in the central region of a galaxy, as well as in star clusters.
In some sci-fi, stars are intelligent and alive, due To their incredibly complex internal structure. However, their lifespan is so long compared to that of most species', that communication is extremely slow, and can only be achieved through a careful time-fusion. This involves the stretching and contracting of space-time. While verbal communication is impossible without time-fusion, it is possible to feel a star's emotions without it through empathic abilities.(The Chaos Chronicles)