The Hertzsprung-Russell diagram is shown above,
(the stars have been given very rough colours to give you some idea of the
temperatures, blue being the hottest, red the coolest.)

There are 3 main types of star.

1) Stars on the main sequence, this is where stars spend most of their life.
They range from the highly luminous and hot blue (but short lived) stars of
spectral class O and B like the stars that populate the Open Cluster M45 to the cooler,
smaller and less luminous stars which are significantly less luminous than our sun.

2) Red giants and supergiants - These stars are very luminous, for example,
Betelgeuse in the constellation Orion is in this group and is one of the brighter stars in our galaxy.
However, they have relatively low surface temperatures. These stars have
left the main sequence and are 'burning' helium and then carbon-oxygen.
The core becomes very dense and has to produce more energy to stop itself
collapsing, hence, the hugely expanded outer shell. These stars are
unstable and do not spend as long in the red giant phase as they do
in the main sequence. Our own Sun will spend about 700 million years as a red giant.

3. White dwarfes are very dense and hot, but have low luminosity.
A white dwarf may have the mass of our sun but only the diameter of our earth.

The Hertzsprung - Russell diagram gives us information regarding the
life cycle of stars. It can also be used in the calculation of stellar distances.

i.e. - If you know how hot a main sequence star is, the H-R diagram
will tell you its absolute magnitude. Given that you now know its
absolute magnitude and that you can observe its apparent magnitude
in the sky, it is easy to work out the distance of the star.