The diagram of Hertzsprung-Russell is a graphic representation of the star spectrum classification. In abscissas there is the temperature, which is related to stellar class, and in ordinates there is the absolute magnitude.
From the graph you can notice that the stars don't gather casually, but they assemble in well defined zones: most stars are along a trace that crosses the diagram diagonally and is called principal sequence; aloft on the right, with low temperatures but high brightness, there are the red giants; in low on the left, the white dwarfs, not very bright but very warm.
Ejnar Hertzsprung (8.X.1873 - 21.X.1967). Danish astronomer and physicist, he discovered the relationship between absolute magnitude and temperature of the stars.
Henry Norris Russell (25.X.1877 - 19.II.1957). American astronomer, studious of the binary stars, he conceived a method to measure the masses of every of the two stars of the system.