Stars are immense, incandescent spheres of plasma that are bound together by the force of gravity. All stars start their lives as clouds of matter, composed chiefly of hydrogen, which collapses under its own gravity until it is so compressed that reactions start to take place between its atoms. Hydrogen atoms are forced together by gravity and nuclear fusion takes place, with the hydrogen being converted into helium. This forms the major part in a star's life. Further fusion reactions take place forming heavier elements, such as carbon, oxygen and eventually still heavier elements, such as iron.
When you look into the night sky on a clear night, with no moon or city lights to spoil the view, although you may think that you can see millions of stars, it is unlikely that you can see more than two or three thousand. Most of the stars in the Milky Way Galaxy are invisible to the naked eye, but it is estimated that there are at least 100 billion of them. There are approximately the same numbers of galaxies, which means that there are probably at least one septillion (that's 1 with 24 zeroes after it) stars in the known universe.
The length of a star's life depends entirely on its mass. A star like the sun has a life span of approximately 10 billion years. It will eventually become a red giant, then a planetary nebula, which will become a white dwarf and eventually a black dwarf. Stars that are more than three times the mass of the sun form a red super giant, which explodes into a supernova and leaves behind a black hole. This can all happen in a little more than a million years. Stars that are between 1.5 and three times the size of the sun also form a red super giant, which explodes into a supernova but leaves behind a neutron star. These stars have a life span of between 1.5 and 10 million years.
The type of a star is dependent on the original mass of matter that goes into forming the protostar, the name for the initial stage of a star. The protostar collapses to form a T Tauri Star, the in-between phase that has not yet collapsed enough for fusion to take place. When fusion takes place, the star becomes a main sequence star, which is what the sun is at present. All stars spend most of their lives as main sequence stars. In the final stages of their lives, stars become red giants or red super giants before undergoing radical change into red, white or black dwarfs. Neutron stars are the leftovers of supernovae and are about 15 miles across and very heavy, but not quite heavy enough to become a black hole.