Ejnar Hertzsprung and Henrey Norris Russel found a correlation between the luminosity (absolute magnitude) and color of 90% of stars in 1912. When they plotted this correlation, most of the coordinates fell within a narrow upward diagonal band. The scientists called the stars whose coordinates were within this band "main sequence" stars. The men called the luminosity versus temperature graph the Hertzprung-Russel diagram (or, the HR diagram). The HR diagram "tells about structure of stars and how they produce their energy". Sometimes the HR diagram is called the color-magnitude diagram because "the absolute magnitude is plotted versus color". The color of a star is indicative of the star's temperature.
Below are two links. The first link is an article about the HR diagram. The second link is to a colored picture of an HR diagram.