I'll start off by saying this is the 1974 one, no other silly remake or the like. Then I'll say that this is a good film, though it is not a well made film. The cinematography and dialogue are all kind of weird, and when compared to other films, even of the era and the time it just doesn't stand up as well, but the tone of the film, and the way it was made are what contributes to the horror, and what make it such a classic that it can still be watched today and considered good.
So the film starts off by saying it's a true story. The whole film is kind of an attempt to make a film like a documentary, like Blair Witch did so famously in 2001, and as the Coen Brothers said this is what makes the film more realistic, because if people believe it is based off of a true story, they allow for pretty much anything. The story is true in that people were killed, at some point in time, by someone, but all the details and just about everything were changed, so the notice is not one of fact, but one to enhance the fiction.
We have a nice narrator detailing the facts of this "true" account over vague shots of corpses, with flashing in and out to make it unclear. Then we cut to the protagonists of the film, five teenagers who seem fairly normal, if very 70's. After this introduction the crew meets the first psycho of the night, a lovely hitchhiker who upon arriving in their van cuts himself and sets things on fire, this is not appreciated, so they throw him out, though not before he cuts one of the characters, and swears a vague act of revenge in the form of blood on the van.
Then the group heads to the house they were planning to stay at, but the house next door is where the action really starts. Surprisingly one pair of characters is killed off very quickly, with little ceremony, the guy first being smashed in the head, then the girl wandering in to have about the same result. The suddenness of these events is what makes it so surprising, as all deaths in the film, with the notable exception of the leading lady last maybe 10 seconds. This creates the fear, because of the sheer viciousness of the acts which kill these characters, and the speed of them.
The entire film is very dirty, or gritty I guess if you want to go with a more positive word. It is dark, grimy, and has bones almost everyone once the characters reach the house, and doesn't seem to do things too deliberately. It is almost the opposite of a Kubrick or a Hitchcock, or even a Wes Craven horror because of how sudden, grim, and dirty all the actions are, it looks like none of it was planned as opposed to their films where it appears everything was.
The other note about the film is that it is very loud in general, from the sounds of a chainsaw from constant shreaking in the third act, but it's the moments of silence, where all you can hear is the chirping of the crickets or the beginnings of the music that make it truly effective as a horror. There is nothing really comic here, just a pathetic bunch of crazies doing crazy things. On the whole it is effective in doing what it is trying to do, a new kind of horror which doesn't rely strictly on shocks or gore, but on the whole combination of grime and blood and terror to create this whole experience.
On the whole, it's a bloody good time, though it features surprisingly little blood.