The Cabin in the Woods is about five friends--the 'virgin', Dana (Kristen Connelly); the slutty girl, Jules (Anna Hutchison); her buff boyfriend, Curt (Chris Hemsworth); his nice friend, Holden (Jesse Williams); and their stoner friend, Marty (Fran Kranz)--go to a remote cabin that Curt's cousin recently bought. But something dangerous is going on behind the scenes with two men, Satterson (Richard Jenkins) and Hadley (Bradley Whitford), working for somebody that has seemingly a little too much power...
And that's all I'm gonna say. I agree with the others: go into this one as blind as possible, if you haven't already seen it at this point. To call this original is underselling it, I think. The story is intensely unique, and not since Scream has a movie taken its predecessors in the genre and lovingly kicked them in the balls. It takes everything you know of the genre and turns it on its head, playing with tropes and ideas. It's a smart film, a level of subtle meta that makes it more like an Easter Egg hunt of awesomeness than a "oh look how clever we are." But it's still pretty damn clever.
The characters are a big part of this, taking what are basically the dumb teens in every horror film and making you root for them, making you hate when any of them die. They are well written, thanks in part to Joss Whedon and his fantastic ear for dialogue, as well as his character building. I mean, nothing is overly complex about these characters. You're given enough to care, but not too much to the point where it feels forced. Let's not forget the "management" characters, who are also a hoot. There's a darkness to their actions, but it's almost always a light mood, with just that hint of desperateness.
This movie goes places you can't even begin to imagine, and the third act is just balls-to-the-wall insanity. (If excessive blood bugs you... you might want to steer clear.) The only downside is some CGI use, but it's only quick glimpses usually, and only for... things... that need it. Otherwise, it's practical all the way.
The movie is scary, intense, and really funny. It's an edge-of-your-seat kinda flick, and you never know when something is going to happen. The build-up is crazy, with scenes that have you cringing and twisting in your seat because you're expecting something, but most of the time it's just... playing with preconceived notions again. It's smart, clever, and amazingly original. If you're a fan of horror films, The Cabin in the Woods should be at the top of your list.