Fortinbras serves as a foil for Hamlet: While the Danish prince is hesitant and given to making long-winded speeches, the Norwegian is impulsive and hot-headed, determined to avenge his slain father at any cost. In this light, the triumph of Norway over Denmark at the end of the play can be seen as a triumph of firm, decisive action over doubt and prevarication; Hamlet's ambivalence and reluctance to act in the early parts of the play exacerbate the situation within the Danish court and ultimately lead to a massacre which eliminates the entire leadership, ruining any hope the nation has of defending itself against the invasion.
http://en.wikipedia.org/wiki/Fortinbras
