This has been requested of me for awhile as I have a lot of knowledge and personal experience but I've been a lazy shit and never bothered to write this. This won't be comprehensive as there's a lot of trivia in the subject of torture to talk about and I just want to explain what I think is more important to know, at least to kick off the thread.
Ok so to begin with I will say that torture is carried out for a lot of reasons and most forms of torture are just sadism that doesn't have a high success rate in accomplishing anything other than breaking a person for the entertainment of whoever is carrying it out. Torture is not very effective for extracting most kinds of information from anyone who has any real determination to resist giving out said information. If information is what you want it's actually better to give them the VIP treatment and get them to give up information to you because they like you or to just ask or to spy on them or to get them to slip up or to entrance them. Lots of ways to extract information out of people without torturing them. Torture is pretty much acknowledged as really only good for demoralization, psychological warfare, and fulfilling one's sadism. Not much else.
Now there are a few methods of torture I want to cover for different cases. To begin with if you just want to try to create a lot of pain for someone, quick and easy, and make them yelp and give up whatever it is you want from them then you need to target the hands, the bottom of the feet, the stomach, and any sensitive places like this. Bending the fingers back too far for example will create sharp and powerful pain that will cause weak-willed individuals to demand you immediately stop and that they will give into your demands. Prodding or burning the bottom of the feet can cause a lot pain, it's silly that the bottom of the feet have so many nerves, but there's a lot there. Breaking toes also can hurt them a lot. Another area you can target is the teeth, pulling people's teeth out or breaking the teeth while aren't sedated at all, is incredibly painful. It does have the problem though that when you break their teeth the pain isn't just going to stop, they'll be screaming for hours or days, so you won't be able to make demands on them as you can't just stop there pain at that point really. It's also improbable they will be able to talk if you've got them somehow bound up in some way that you can fuck around with their teeth. As for the stomach you can potentially create a lot of pain there if you do things right, but you can also accidentally kill them a little too easily, or knock them unconscious. As such if you just want to make someone submit to you quick fucking with their fingers is a good way. You may also put people into locks and just bend any joint you want slowly, which will cause lots of pain, but you can stop before doing any real damage. I have been targeted in all these areas myself and endured it all very well. Me personally I've had the fingernails of both my middle fingers pulled off backwards… which for me wasn't that big of a deal actually. It's all red underneath there btw once the fingernail is off and it takes months for the fingernail to grow back. Concerning all the areas I've mentioned thus far, some kind of poison I was given really caused an amazing amount of pain in my stomach, for about 2 or 3 days. I don't know what poison was used. I will say that all though all of this is simple and quick to apply in a situation where you've got someone hostage and want to submit them right away.