Fortinbras may be seen as a foil for Hamlet, a foil being a character that is set up as a basis of comparison for another character.  They are both similar in that both men have lost their fathers and both are attempting to fulfill the duty of avenging this death. 

To this point, Hamlet has been ineffective in getting revenge, taking many detours to his original plan.  He knows this flaw in himself as proven when he berates himself after watching an actor perform an emotional scene and comparing the actor's abiity to portray an emotion when Hamlet cannot act upon a very real necessity.  

Fortinbras, on the other hand, has been very methodical in his plan.  He has deceived Claudius and obtained permission to march his troops across Denmark under the pretense of attacking Poland.  Hamlet, on his way to England, encounters Fortinbras' troops and cannot understsand why such a vast movement to obtain a worthless strip of land.  

Again, he compares his own sense of duty to those of the actor and now Fortinbras, both of whom have less a need to at than Hamlet but both of whom act nonetheless.  Hamlet admires Fortinbras for this.