What is Hamlet all about ? What are the themes and messages throughout the play ?

In Hamlet, there is an act of
betrayal by Claudius. Then the entire play revolves around young Hamlet's desire to
avenge his father's death:

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Hamlet is often called an "Elizabethan revenge
play", the theme of revenge against an evil usurper driving the plot
forward.

Hamlet
is a play that causes young Hamlet to not be able to rest until justice is served.
Hamlet spends part of the play questioning whether or not his Uncle Claudius could do
such a thing as murder his father. Once Hamlet is convinced that Claudius has indeed
murdered his father, he spends the rest of the time plotting his retaliation for the
murder of his father. The end result is a tragedy:

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But Hamlet is far more than an outstanding
example of the revenge play. It is, to begin, a tragedy in which the attainment of
justice entails the avenging hero's
death.

The theme or message
is that revenge itself is deadly. Hamlet causes his own downfall by his obsession with
revenge. By plotting Claudius' death, Hamlet is stirring up Claudius' natural reaction
to protect himself. Ultimately, revenge kills Hamlet. His death is
inevitable:

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Life inevitably yields death and a wormy grave,
and its occurrence cannot be foreseen or
avoided.