"Silent Hill" is a series of video games, which they also made a movie for a few years back. They are survival horror games, and Silent Hill itself is a town where many bad things happen, haha. The plots are pretty involved, so it won't make sense for me to try to explain it (you can read here, if you want:
http://en.wikipedia.org/wiki/Silent_Hill), but I really love (most of) the games, because they are the perfect mix of actual story, ambience, and gameplay - I love anything with a psychological bent, and these games have that in spades.
They did start going downhill after the 3rd one, but the awesome that is Silent Hill 2 makes up for everything in my book.
The movie they made was all right - it sort of follows the plot of the first game, but they changed the main character to a girl. In the game, it was Harry Mason trying to find his daughter. In the movie, it's a mother instead. Maybe the filmmakers thought audiences wouldn't identify with a guy running around after a lost child like that? I don't know.
They also took a bunch of the monsters from Silent Hill 2 and stuck them in there. They are scary monsters and interesting, so I understand wanting to put them in the film, but they don't belong.
The monsters people see when trapped in Silent Hill are sort of like reflections of their own psychological screwiness - for example, the "sexy" nurses you see in the movie are from Silent Hill 2, where the main character, James, is searching for his dead wife. They represent his sexual frustration - there are nurses in the first Silent Hill, but they aren't well-endowed or "sexy" like they are in SH2, and the filmmakers went with the sexy ones anyway. Pyramid Head is also one of James' monsters, as is the "Lying Figure," the first monster the mom and the cop encounter after they realize they can't leave town in the movie.
I love me some Silent Hill like a fat kid love cake.