One of the best movies I've ever seen. The only other Guillermo del Toro movies I've seen are Hellboy and Pacific Rim, and this movie upstages those two in every way.
The faun, portrayed by Doug Jones and voiced by Pablo Adan, is one of my favorite characters. He acts as a shepherd for Ofelia to return to her true home as the princess of the underworld. In our world, we have Captain Videl, a sadistic, psychotic captain in the Spanish military facing a group of insurgents, who are against the new Spanish government established after the second World War. This brings up a second interesting aspect of the film, that it uses this time period, and that Ofelia's world is like hell but looks idyllic, while the idea of returning to the underworld seems evil but is actually good.
Now, my absolute favorite part of this film is the Pale Man sequence. The second task the faun gives Ofelia is to retrieve a knife from a child eating demon. The faun warns Ofelia not to eat any of the food on the table. Ofelia gets the knife with the help of three fairies, but eats a grape, waking up the Pale Man. She is chased through the Pale Man's lair back to her room in Videl's house.
Now, this is not a family or kid's film, like the David Bowie movie, Labyrinth. There are scary scenes, especially the Pale Man scene. I wouldn't suggest showing this to your kids until they're teenagers, or aren't easily scared.