According to the experts, there are three sublime emotions employed throughout the use of Dark Fiction: Terror, Horror, and Gross-out. All three are versions of fear in some sense or another—that delicious feeling that many well-adjusted contributing members of Western society have craved—but it is the subtle differences in a writer's approach towards those emotions that mark what family a story belongs to. Namely, if it is true Horror, a Tale of Terror, or a yarn built specifically for the blood-soaked Gross-out effect.
The first order is Terror, in which the story attempts to convey upon the reader that dread or anticipation of a terrible event about to happen. Terror is most often employed with ambiguity, with the monster obscured, and it stirs a fight-or-flight response in the reader often recognized as the fast-beating heart and sweaty palms. It is the moment before the shroud comes off. In this, the sensation of Terror can be compared to suspense, in that it occurs just before the abominable event happens. The larger family of Tales of Terror often include Gothic fiction, Weird Tales, Ghost Stories, Thrillers, and occasionally those cuddly fireplace mysteries your aunt reads.
Next, there is the illustrious Horror, which signifies an emotion that occurs after the awful deed has taken place. The anticipation of the event is replaced by awe and fear of the event itself—it is radiation poison and mass graves after the war, it is the shock of what lies hidden in the attic. Horror stories are what most modern Western audiences are used to, and it can be seen reflected in most Mainstream, Domestic, Creature and Cross-Genre Horror. It can also be called (in some circles of people who want to talk about Horror but who don't want to look like they're talking about Horror) the Grotesque.
Gross-out is a relatively new addition to the pantheon of dark feelings, and has been oft maligned for being the baser of the three. Also known as Body Horror, the Gross-out exploits humanity's fear of death and dismemberment by displaying both in the most creative ways possible. It derives from the emotion of horror, but it emphasizes the salacious nature of revulsion and bodily disgust. Most famously in this category are Splatterpunk, Serial Killer fiction and many forms of Bizarro. Despite it's similarities to the Grotesque of old, Gross-out gains its own place on the Dark Fiction family tree, because it raped that family tree's bleeding eye sockets until the rooty thing gave it what it wanted.