Even though Fortinbras is a character who remains unseen for most of the play and only enters after Hamlet's death in the very final scene, it is clear that Shakespeare draws many comparisons between these two characters, and clearly indicating the way in which Fortinbras acts as a foil for Hamlet in the same way that Laertes acts as another foil.
Let us first focus on the similarities between them. Both have lost fathers at the hand of somebody else, as Hamlet lost his father thanks to the murderous intentions of Claudius, and Fortinbras lost his father thanks to Hamlet's father, who killed him in war. Both therefore are in the position of being sole heirs or princes to a kingdom and also needing to revenge the death of their father.
However, this is where the similarities end, because, throughout the play, even though he only appears in the final scene, Fortinbras is shown to be a constant rebuke to Hamlet in the way that he actively pursues his revenge and conducts himself as a prince. Note how he is refered to in Act I scene 2:
Even though he is not King of Norway, he is actively doing everything he can to revenge his father and to gain back the land that was lost to Denmark. This is of course very different from Hamlet's prevarication and procrastination to the situation that he faces, where he seems to deliberately spend as long as possible trying to decide whether Claudius is guilty or not before finally moving on to do anything about it. In addition, note that when the final showdown happens, it is instigated by Claudius and Laertes rather than Hamlet himself. He appears to be a rather passive character in so many ways, especially when compared to Fortinbras, who is shown to be active throughout the play and committed to regaining his lost lands and revenging his father.