What is this Strangers movie about?

I saw the preview for the 2008 horror movie called the strangers. it shows some scary looking people with masks on swings and stuff with like axes. What is the plot? i don't get it. it looks really scary. also, for anybody who saw the movie Prom night in America there was another scary commercial after that were an apartment is evacuated because some like demon lady is in their. I think she's possesed but she has fangs and stuff. in the preview you see like night vision as if somebody is holding a camera walking through a hotel or something. they find a case with a camera. its scary looking too, thanks!