A star is a sphere of plasma that is held together by gravity. Stars come in many different shapes and sizes. The size of a star is totally dependent upon the size of the molecular cloud it formed from.
Most stars that astronomers have discovered are on the main sequence because stars spend 90 percent of their lives on the main sequence. A star on the main sequence fuses hydrogen into helium. The lower the mass of the star, the longer it stays a main sequence star. For example, our Sun is 4.5 billion years old and it is still on the main sequence. It will remain on the main sequence for about another 4 billion years. If a star is much more massive than our Sun it could spend as little as a million years as a main sequence star.
After a star moves off of the main sequence it becomes a red giant. It swells an enormous amount and it becomes much cooler than it was when it was on the main sequence. A star like our Sun will stay on the main sequence for about a million years. At the end of a stars life, depending on its mass, it will become a white dwarf, a neutron star, or a black hole. A star like our Sun will become a white dwarf.