There are several kinds of horror. The one we know all too well here in America are the "monster in the house" stories. Ones where some external threat, often back from the dead or from outer space, and it's either fight or die. Then there's the absolutely bizarre: the works of Junji Ito come to mind. In those, there's some terrible force at work and the horror comes from the total perversion of life as we know it.
Both of those can be scary, but I'm largely able to enjoy them for the story themselves -- or for the comical action. There's one other type, though. It gets under my skin with characters that you can relate to, feel comfortable with, and then see glimmers of pure evil in them.
That's what Higurashi: When They Cry achieves. The only other title that gets to me like this is Silence of the Lambs.
So follow me after the jump and you'll see why Higurashi is like this, and how it differentiates itself from the anime, which Pedro has already recommended to you.
Higurashi: When They Cry
Creator: Ryushiki07
Publisher: Yen Press
Release Date: November 2008 (Vol. 1)
MSRP: $10.99
To shortly summarize Higurashi, it follows Keiichi Maebara as he moves to the quiet little town of Hinamizawa. The year is 1983, and life is almost idyllic. He's quickly made friends with girls of all ages within the town, and everything seems perfect. Except when he tries to find out what happened the other year with a supposed murder in relation to a dam construction. Everyone stonewalls him, and in a manner that's almost disturbing.
From there, it's a journey into hell.
The horror comes slowly, but you know it's there. It creeps in, springing upon you at the calmest of moments. You turn the page, and suddenly Mion or one of the other girls has taken over this possessed look about them, suddenly able to see through Keiichi's defenses. Emotion is in the eyes for the Japanese, and the art showcases that very clearly. Even now, when I flip to see some of the panels, a chill runs down my spine. See what I mean:

This imagery is backed up by an incredible sense of pacing. The story, as I mentioned earlier, works by lulling you into a sense of security with low-tension scenes. Yet, as you see Keiichi interacting with these girls, you know something is wrong, and the manga builds up a sense of anxiety within you as you read on. When details are revealed, details are piecemeal.
Like the anime, the manga goes through numerous arcs, meant to explore all the different possible paths in the original PC game. As you go through more and more arcs, you begin to unravel all the details of what the "truth" is in Hinamizawa, and what all the characters are actually like.
I'm still trying to figure it out. Yen has just finished publishing the second arc of the story, and there's plenty left to go.
One of the things that need be mentioned in the difference between the anime and manga: the manga leaves much more up to your imagination, allowing you to fill in blanks with your own graphic images of how things would progress. In some cases, it will show more graphic imagery than the show, but the utilization of blackened pages is great, in my mind.
If you don't want to sleep for a week, then pick up Higurashi right away. It's fantastically creepy, disturbing and memorable. Higurashi has easily taken the slot as my second favorite manga, right behind Gunslinger Girl, and I can assure you I will be following it doggedly.
& I'm glad you mentioned the difference between the manga & the anime since I was hoping they wouldn't be the same, thus kind of ruining the atmosphere since I'd know what would happen.