the movie was "okay"..but I am one of the people who needed clarification from a friend of mine, who is a Silent Hill guru. So here goes, if I remember everything correctly:
*SPOILER*
The different worlds are from 2 gods of silent hill, Samael (bad) and Metatron (good), plus the messed up girl's version of the world. The gods are constantly fighting, and when one has the upper hand, the world shifts. Samael is obviously the bad Silent Hill, Metatron is when SH is "safe", and the girl is the hellish version. The crazy lady in the movie, is bad in the game I believe, she's trying to bring Samael into "our" world by use of the girl, her daughter, by means slip my mind. I believe that the hero(ine)'s daughter is one half of the crazy lady's daughter, and the other half could be the tormented girl, i believe. Pyramid Head is on Silent Hill 2, and is supposed to be some kind of manefestation of SH2 Hero's mind, something like that. Why they have him in the movie, I have no idea, but there's more than one of them in the SH2 game, so there could be more in the sequel. and everyone who is in Silent Hill has been there in the past, when it used to be a resort town, and the town is trying to get them to come back for some reason, via nightmares and trickery, stuff like that. The rest of it escapes my mind, and if I am wrong, I deeply apologize, but it was a while ago. Just tryin to help
*END SPOILER*