Stars start as Main sequence stars and evolve as they age. This process can be seen by plotting a star cluster on the HR diagram.
Early Life of a Cluster
Clouds of gas and dust collapse to form Main Sequence stars.
Notice that many low mass stars are still forming (and joining) the lower main Sequence.
Childhood
As time goes by the remaining low mass stars join the Main Sequence while
high mass upper Main Sequence stars leave to become Supergiants.
The Teenage Years
Most very high mass stars have become Supergiants or have died in supernova explosions.
The Adult Years
Nearly all high mass stars are gone. Medium mass stars begin to leave the Main Sequence and become Giants.
Middle Age
More medium mass stars leave the Main Sequence and become Giants.
Some of the older Giants collapse to become White Dwarfs.
Old Age
All of the medium mass stars have left the Main Sequence, became Giants, and many have collapsed into White Dwarfs.
Low mass Main Sequence stars do not become Giants, but die and become White Dwarfs.
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