What is the role of the Ghost in the play?

In Shakespeare's Hamlet, the Ghost
serves as catalyst for the plot.  When he tells Hamlet that his father was murdered and
charges Hamlet to get revenge, he ignites the action of the play.  This is a revenge
tragedy, and the Ghost starts Hamlet on his revenge.

The
Ghost also contributes to Hamlet's predicament and his lack of certainty.  Because the
Ghost's identity is in question, Hamlet needs corroboration before acting on the Ghost's
story.  The killing of a king is no small matter.  This leads to Hamlet's pretending to
be "mad," and his setting up the play-within-the-play to prove to himself that the Ghost
is telling him the truth.  According to Elizabethan tradition, the Ghost could be the
ghost of Hamlet's father, but he could also be a demon disguised for the purpose of
bringing chaos to Denmark. 

The Ghost, then, contributes to
themes of revenge, madness, acting and seeming, and to the issue of why Hamlet waits or
delays.

The Ghost also provides a concrete image of
Hamlet's father, giving concreteness to the person Hamlet opens the play mourning for,
and provides exposition by revealing details that occured before the
opening. 

Of course, with his later appearance in the play,
the Ghost also, among other things, and perhaps inadvertently, draws attention to
Hamlet's obsession with his mother.  Hamlet is at least as upset with his mother's hasty
remarriage as he is with his father's murder.  Hamlet is obsessed with his mother's
sexual relations with Claudius.  And the Ghost rebukes him for
it.