Stars are extremely massive balls of gas in space held together by their own gravity, and are classified according to spectral analysis.
Spectral classification is determined by observing stars and categorizing them based on how their light is perceived (the perception of light waves determines what color they are perceived to be), as well as the chemicals that comprise their chromospheres. Under spectral classification, stars are placed in categories of O, B, A, F, G, K, and M; along with that, each category has subcategories which describe more specific star types. A good mnemonic device for remembering the spectral class system is "Oh, be a fine girl - kiss me".
Class O stars are the hottest and largest observable stars. They are mainly extremely massive blue giants, which put out more than one million times the amount of energy as the Sun. Their blue color comes from being extremely hot. They convert hydrogen to helium very quickly; because of this, they burn out at a very fast rate. Habitable planets can't form around Class O stars, because of the photo-evaporation effect. Class O stars are so hot, they would strip any close-by planets of their atmosphere.
Zeta Puppis (or Naos) is a Class O star, with the subcategory of O5 laf. Zeta Orionis (Alnitak in Arabic) is also a Class O star, and is the brightest star of this category that can be seen by the naked eye.
Class B stars are blue giant stars that are more common than the Class O type. Class B stars are noted for their intense brightness and tend to cluster together. The brightest stars of the constellations Orion, such as Rigel, and Pleiades are Class B stars.
Class A stars are very common, with as many as 1 in 160 nearby stars being of this category. Class A stars are white or bluish white, which means they are very hot. Stars of this variety have the strongest hydrogen lines out of any star class. Sirius (the Dog Star), which is actually a binary star system, is the brightest star in the night sky and is of the Class A variety.
The hydrogen lines of Class F are fainter than Class A stars and labeled by the Harvard spectral classification system as "medium". Class F stars comprise of 3% of all main sequence stars, which makes them a very common occurrence in our stellar neighborhood. Arrakis, or Mu Draconis, is a well-known Class F star.
Earth's Sun is a class G star. Class G stars are extremely common and yellow in color. When scientists look for stars that might have Earth-like planets around them, they look at class G stars. The reason for this is because stars of this class are of a medium temperature, mass, and age, which is favorable for the prospect of life on surrounding planets.
The Alpha Centauri system is the closest star system to Earth and is home to a Sun-like star, Alpha Centauri A. Class G stars can vary in size somewhat; Alpha Centauri A is (on stellar terms) slightly bigger than our Sun.
Class K stars vary greatly in size amongst each other and have weak hydrogen lines. Stars of this variety are orange in color, with red giants falling into this category.
Alpha Centauri B and Aldebaran are both Class K stars. Alpha Centauri B, a close neighbor, is slightly smaller than the Sun, whereas Aldebaran (also known as the Bull's Eye from its size and location in the Taurus constellation) is extremely massive and large, being 44.2 times larger than the Sun and far brighter.
Class M stars are the most common stars in the sky. They range in description from red dwarfs to supergiants, such as Betelgeuse. Earth's closest neighbor, Proxima Centauri, is a Class M red dwarf. Class M stars are noted for weak hydrogen lines. Brown dwarfs belong to subcategories of the M star class.
The one thing that all stars have in common is that they are born, they live, and they die: much like human beings. At the end of a star's life, they collapse onto themselves and become a black hole.