There are a number of physical characteristics of stars that provide important information on the lives of stars, including mass and age and luminosity and temperature. The progress of a star's life is predestined by its mass, because ultimately the mass determines how much energy the star can produce and how quickly it will do so.
Two astronomers of the early 20th Century, Ejnar Hertzsprung and Henry Norris Russell, discovered an important observational means of comparing different stars with one another. They found that when you plot the brightnesses of individual stars versus their spectral type or color on a graph, the stars lie within well-defined areas within the graph. A star of a given brightness could only lie within a certain range of colors, and a star with a given color could only lie within a certain range of brightnesses. More observational and theoretical research showed that the color-magnitude diagram or Hertzsprung-Russell diagram was a snapshot of the evolutionary states of the stars plotted within the diagram. Stars would be found in different parts of the diagram depending upon their masses and their ages. As a star gets older, it changes in brightness and color in a very predictable way, and that stars of different masses change in very different ways.
Stars would be found in different parts of the diagram depending upon their masses and their ages. As a star gets older, it changes in brightness and color in a very predictable way, and that stars of different masses change in very different ways. From the location of a star on the graph, the luminosity, spectral type, color, temperature, mass, chemical composition, age, and evolutionary history are known. The Hertzsprung-Russell diagram, or H-R diagram, is the periodic table of the stars.
When we talk about stars, we often refer to them based upon their position in the H-R diagram. For example, we call stars that are still burning hydrogen in their cores main sequence stars, and will often refer to stars younger and older than main sequence stars as pre- and post-main sequence stars. Stars that have evolved well beyond the main sequence are often on the red giant branch of the H-R diagram, or might be asymptotic giant branch stars. All of these are stages of stars' lives, and the classifications help us to put them in context within the broader picture of stellar evolution.