To have very good social skills, you have to care about having good social skills. You also have to enjoy and take pride in having good social skills. And you have to have very thick skin. INTPs most often don't have these traits. We can certainly develop them to a very respectable degree, but no one will consider us paragons of interaction.
What exactly does it mean to have good social skills?
The same thing it means to have good driving skills. You have to have enough experience and practice that you can operate with confidence and success in any normal situation, without making a mistake that will cost you. As in driving, others can compensate for your errors, which is why most errors aren't fatal. You can come across as rude or courteous, quiet or boisterous, but what matters is if you are smooth and confident.
What does it mean to be socially inept?
To be socially inept is to lack good social skills. Lack of confidence, experience, and practice, and an inability to judge the effect you are having on others, all can make you socially inept. People with good social skills can certainly put their foot in their mouth, but they will realize it quickly and come up with an appropriate way to correct the situation, fast. People without these skills may not even know they have offended, and may not care.
Whose criteria is used to judge this?
The majority of the people in the culture you are in. Extraverts. ESxJs. The rest of us get to nod.
What are some social skills or tactics/strageties have you noticed or developed in yourself or others? What makes these work and in what context would they best be applied?
Well, some people have been discussing comebacks and putdowns. I suck at these, and hate them. In the US, they do seem to be required to be considered socially deft, but I will never get it, so I don't even try. My strategy is to ignore it if possible. If it is an insult I just can't abide, I tell them so and let them think I am thin skinned.
Others brought up personal relationships. INTPs are loyal and dedicated friends and lovers. We handle betrayal very badly. While we don't develop relationships fast, when we do develop them, we work very hard to maintain them. We aren't considered socially deft because of it, because most people don't value the relationships the same way we do. But we are certainly desirable friends and mates, to those who realize it. I think social skills (looking like everyone else) and relationship skills (getting along with those who matter) aren't exactly the same thing.