This is a graph that represents the stars of our universe. The vertical axis measures luminosity, i.e. how bright a star is. The horizontal axis represents surface temperature. Be careful though because the temperature does not get hotter as you go from left to right or from blue to red, it gets cooler. Blue is hotter than red and the horizontal axis starts at the left hand side with the hottest stars.
So stars in the top left hand corner are very bright and very hot. Stars in the top right hand corner are bright but not as hot.
The line going through the centre of the graph is called the main sequence. Our sun sits somewhere near the middle of that line. Middling brightness and temperature.
I heard about the HR diagram when I watched a series of astronomy lectures from Professor Robert Nemiroff. I downloaded the lectures from iTunes (search for Introductory Astronomy, PH1600). The actual classes were given in Michigan Technological University in Houghton, Michigan. Nemiroff also runs a website called Astronomy Picture of the Day or APOD.