A star is born through gas and dust, out of nothingness into a collapsing cloud that becomes so dense that it becomes a proto-star. When nuclear fusion commences, this is the beginning of a star.

http://space.about.com/od/stars/tp/What-Are-The-Different-Types-Of-Stars.htm

There are many star types: Brown star, Neutron star, Black hole, White dwarf, Red giants, and variable stars. The thing about stars is that they exist or they fail. The size of a star depends on how much mass they can obtain through the gas and dust around them. Stars are interesting in that they grow to become a red giant star (the state of using up all their hydrogen fuel) and either become a black hole, neutron star or white dwarf all depending on their size. Our sun 1 solar mass, the biggest star that is suppose to exist is 150 x solar mass. There are anomalies in the universe though where two or more stars merge to become one big massive star which is the case of a red giant star called VY Canis Majoris which is 2600 solar mass which is suppose to die soon and become a black hole because it has become so massive. The balance of a star is very delicate in that it can become a white dwarf and neutron star or a black hole. If it becomes a remnant star (white dwarf or neutron) then it has a chance at reigniting nuclear fusion in the future.

http://www.universetoday.com/13507/what-is-the-biggest-star-in-the-universe/