A stellar nebula is a cloud of gas and dust, clumped up over a few millenia. Gravity causes the denser material to clump up and contract. As it does this, the material starts to heat up and spin (as explained by the law of conservation of angular momentum). When enough material has been gathered, a Proto-Star is formed.
A Proto-Star is a star in it's infancy. With a surface temperature of around 2000K, it does not develop into a Main Sequence star unless it has a mass of around 8% of our sun's mass. If it does, with temperatures of around 15,000,000K, fusion starts. Hydrogen begins to fuse into Helium. As fusion starts, an equilibrium is achieved between gravity forcing the star to contract, and gas and radiation pressure forcing the star to contract. The star is now in Main Sequence.
The star begins to shine as it fuses Hydrogen into Helium. This process takes millions of years. Our sun is currently an average-sized main sequence star, with a surface temperature of around 5773K. When a Main Sequence star has burned all available Hydrogen, it becomes a Red Giant or a Red Super Giant, depending on mass.
4.Red (Super) Giant