This is a sub-genre of horror that's come into being over the last five or six years. One of the earliest movies that was categorized as torture porn was Saw. What usually happens in a torture porn flick is someone is captured by the bad guy, and then brutally killed, usually slowly, and with no punches pulled in terms of showing things on screen or showers of blood. After one or two characters are slaughtered, it usually switches to the "hero" character escaping the bad guy after being captured too.
Now here's the thing about torture porn: it's not new. Really, it's not. It's been around for a long time. You can go back to things as early as the original Texas Chainsaw Massacre, I Spit on you Grave, and others that are usually categorized as slasher or suspense flicks and find definite elements of torture porn in them. That's because really, at its core, torture porn is a blend of the slasher flick and the standard abduction suspense flick.
At its heart, torture porn is really all about suspense, and that's where the real guts of its horror elements come from. Of course, while the torture scenes happen, the audience gets a gut sense of "ewwww", but that type of scare is and always has been fleeting. Where the real horror elements of the genre happen are when the hero character starts making his/her escape. Why do these suspense elements work differently than your basic suspense flick? Because we've seen, in detail, exactly what will happen to the main character if he fails to escape from the killer.
That vital difference is what sets the genre apart from the rest of the horror genre. In most suspense flicks, the hero has to escape the villian because the villian will kill him in undefined bad ways, and the mechanics are left to the audience's imagination. Frankly, most people don't bother thinking in detail about what those bad things are, so the sense of danger is fairly non-specific. By showing you that the hero will, for example, be fastened to a table with screws, have their eyelids cut off, then their entrails extracted one by one and shown to them, you feel much more strongly about the hero's escaping.
So how is that any different than slasher flicks, you say? Tone. Most slashers, especially in the later years of the genre's prominence, played things for laughs. The whole thing was about tongue in cheek, formulaic violence. You were never rooting for the characters, you were rooting for the killers, and cheering as they took out the victims in more and more ridiculous ways. Moreover, few of the kills were given a lot of screen time. Yes, Jason just impaled a couple with a harpoon, but the whole thing happened in less than 30 seconds. Your average torture porn killer is utterly unsympathetic, and the killings can take many minutes. These kills are not intended to be funny, and because of that, you automatically sympathize with the victims.
Another vital difference is that it's way more common for torture porns to have bleak endings. Slasher flicks almost always have the last girl emerge victorious (or at least not dead). In tortuer porn flicks, the "hero" has at best a 50/50 chance. Because of that, you can't ever really relax into "well, I know she's going to live".
The problem that the torture sub-genre has is that it's very difficult to make a good one. It's extremely easy to fall off into just showing tons of blood and violence, and ignoring the vital elements of suspense. A perfect example of this is Hostel 2. The first Hostel was a shining example of how the genre can be extremely effective. In the second, the director opted to keep piling more and more bodies up on the screen, and worse, actually tried to make the killers sympathietic. Unfortunately, he lost sight of what made his first film so good, and wound up making absolute dreck.
So what are some examples of good torture porn? The first Hostel is an excellent example, as is the first Saw. The subsequent movies in the series have actually done an admirable job of not sucking, though they're getting too bogged down in their own mythology to really qualify for this list. The first Texas Chainsaw Massacre fits here, though it's almost completely bloodless. Some of the best examples of the genre actually come from France, with High Tension and Inside being amazing.
Now, this genre is absolutely not for everyone. They're incredibly graphically violent, and the good ones have an intensity that simply doesn't let up for the entire movie, which is what makes them so good. You don't so much watch these things as you experience and survive them. You really have to get past the ultra-violence to appreciate them as something other than gore for gore's sake. Even for horror film fans, these things can easily get written off as pure gore. But if you've got the stomach for them, and are willing to stop and consider what the film's actually doing, the genre has a surprising amount of depth. If anyone's interested in a screening, let me know.