Dionysus and Pentheus are classic example of foils. Dionysus is a wild god of nature. Pentheus is a law-abiding mortal of the city. Dionysus favors crazy revelry. Pentheus is into law and order. Dionysus represents unbridled primal energy. Pentheus represents the world of reason. The two characters really couldn't be more different.
The contrast between these two characters becomes symbolic of a conflict that perhaps exists within all human beings: the rational vs. the irrational. The idea is that we've all got a little crazy bubbling underneath the surface, a Dionysian world of pure instinct and emotion that our logical Pentheian side struggles to control. Maybe within us all there's a Dionysus and Pentheus at war.