Obviously these things are cultural, but I think there are tiers of questions that can be asked to different tiers of people.
General ones, like 'where do you live?', or 'how do you know the hosts?' are fine in most circumstances, as they are opportunities for the askee to expand on the questions and open a conversation. Where a lot of people go wrong is forgetting that the asker shouldn't persist if the answer seems to be brushing it off. 'Oh, we've known each other for years' should be enough. Not everyone wants to say 'we met in jail', or 'in the waiting room of the STD clinic'
. A skilful conversationalist will pick up on a reluctance to go further and change the subject, but a nosy parker will press until they get a reply.
With friends and family the same rules apply, but the range of acceptable questions is broader. 'How did you get on at the doctor's?' or 'did the interview go well?' is ok if you've been told there was an appointment, but if the answer is 'oh, fine thanks', or 'I'm feeling better now', it's time to back off, unless you've already been involved in conversations about the ailment or job hunt, and are sure that the person doesn't mind you knowing (whatever the outcome), and that there are not others listening who might not share your levels of inclusion.
Questions about relationships and achievements are on less certain ground. 'Are you in a relationship?', 'Did you pass your exams?' 'Are you going to have another baby?' are all things that IMO should be treated with extreme caution. It might be ok to ask a close friend or a close relative, but it's important to judge that closeness carefully - not everyone wants to talk about personal things even to their nearest and dearest, and knowing when to stop is the important thing - people will tell you what they want you to know.
Money, politics, religion are always assumed to be no-go areas, and they are at a casual meeting, such as a wedding; but much depends on the circumstances. I've been to parties where all three have been discussed, and others where they caused conflict - again it depends on the people and how well they can accept differing viewpoints as well as knowing when to stop pushing.