H-R Diagrams

From the Wikipedia page on H-R Diagrams

Put simply, the Herzsprung-Russel Diagram is a graph that shows the relation between a star's luminosity and its effective (surface) temperature.

The diagram increases in luminosity going up the y-axis.  It increases in temperature moving left along the x-axis.  "Why does the x-axis increase to the left?" you might ask (because I definitely did).  It increases to the left because the x-axis used to be B-V color instead of luminosity, and the more negative B-V values corresponded to higher temperatures.

The diagram breaks down into three basic categories.

Main Sequence

Generally speaking, stars spend most of their lives on the main sequence.  They leave or evolve from the main sequence by cooling down (but not getting smaller/losing mass) and becoming a giant.  They can also lose enough energy that they condense and become a white dwarf.  The ultimate fate of a star depends on its mass.

White Dwarfs

These are in bottom, left-hand side of the graph because they're really hot, but not very bright.  They evolve from low-mass stars who move off the main sequence because they run out of hydrogen to burn and don't have enough energy to resist collapse.

Giants

These are in the top, right-hand side of the diagram.  When high mass stars run out of hydrogen to burn, they puff up and become giants.  In the process of puffing up, they lose some of their temperature.