“The 3 types of terror: The Gross-out: the sight of a severed head tumbling down a flight of stairs, it's when the lights go out and something green and slimy splatters against your arm. The Horror: the unnatural, spiders the size of bears, the dead waking up and walking around, it's when the lights go out and something with claws grabs you by the arm. And the last and worse one: Terror, when you come home and notice everything you own had been taken away and replaced by an exact substitute. It's when the lights go out and you feel something behind you, you hear it, you feel its breath against your ear, but when you turn around, there's nothing there...” - Stephen King
I figured it worth the time to take a look at some of these a bit more in-depth. I'll tell you right now, that was a mistake for this part. Blech.
The gross-out is exactly what you would think. All the most disgusting, revolting things that make your stomach turn over. When a head explodes, a fountain of blood gushes out of a random orifice, someone throws up, that's all going for the gross-out. It's not trying to catch your breath, or send shivers down your spine. It's going for that gut reaction that makes the bile rise up in your throat a little bit.
Now, there's nothing wrong with that. It's even the basis for genre's such as Splatterpunk and Torture porn. So there's certainly a market for it, and there are people that appreciate it.
The SAW series is pretty good for an example of the gross-out. While the story itself stands up pretty well, and has an incredible amount of tension, it doesn't look away when the string snaps and people's heads are ripped open.
Of course, with whole genre's based around the gross-out, you also end up with things like COWS (read at your own risk). stories which seem to be written or told just to see how fast or how many times they can make you lose your lunch.
Of course, the gross-out isn't just a genre. It's a tool. It pops up in a lot of great horror. Stephen King himself uses it scattered through his books.
Jack Ketchum's Off Season is rather famous for being "raw, and disturbing", but it's only in the last third of the book, with it's descriptions of cannibalism and perversion that it gets to that point. The rest of the book remains a regular horror thrill ride of a group of friends fending off a savage attack by the cannibal clan.
Even my own work, Class 5, has scenes which go for the gusto of gross rather than the tried and true method of leaving most of the details to the reader's imagination. The scene of a young boy partially devoured, for example.
So what is the difference between a gross-out and just your average violence? It's more than just not turning away at the pivotal moment. It's about taking the moment and reveling in it. Going into more detail than story or description needs. It's when the author gives up on Terror or even Fear and just goes for triggering revulsion.
For example. When the alien grabs your hapless victim by the head, the reader needs to know that it crushed the human's skull like a styro-foam cup, rather than caress the man's hair. The reader doesn't really need to know that its thumb pushed in through one of the eye sockets, mashing the orb like a grape before the rest of its fingers pressed in, shattering the skull like glass, shards of bone sticking out between the fingers as blood and brain fluid ran down the remains of the man's face.
But it is nice to give them that detail once in a while. Just for fun.