The Hertzsprung-Russell diagram is a graph that plots the temperature of a star against its absolute magnitude or luminosity. It was created by astronomers Ejnar Hertzsprung and Henry Norris Russell about 1910, and can be used to chart the life cycle or evolution of a star.
Most stars, including the Sun, can be found in the main sequence of stars - a prominent band running from the top-left to the bottom-right of the diagram. In this main sequence we find that the hotter a star is, the greater its absolute magnitude. Both these factors are determined by the stars mass. However, giant and supergiant stars tend to expand in the final stages of their lives and become cooler as a result; but because they are so big, they are very bright, and thus appear above and to the right of the main sequence. White Dwarf stars, on the other hand, are extremely hot and dense, but because of their small size, they are not very luminous. As a result, they can be found below and to the left of the main sequence. In general, stars will spend most of their life (~90%) on the main sequence before evolving into a giant star for the remaining 10%. Following that, they will either go supernova or become a white dwarf.