The theater of Shakespeare's time was crowded with examples of revenge tragedy. Shakespeare was one of the genre's most prolific playwrights. Four elements common to all of these plays are: murder, a ghostly vision of the murder victim, characters afflicted with madness, and a concept known as meta-theatricality.
Though these four are present in all of Shakespeare's major revenge tragedies, they require a little further explanation for an understanding of their role in the literature.
For example, the murder victim's ghost is also responsible for suggesting a revenge plot, which generally leads directly to the madness required of a true revenge tragedy plot. As for "meta-theatricality," that's just a fancy word for a "play within a play." Think of the play put on in Hamlet as an attempt to accuse a murderer. Such meta-theatrical moments were common in almost all Elizabethan examples of revenge tragedy, including William Shakespeare's work.