Interviewers are legally barred from asking questions about your religion, color, race, national origin, marital status, sex, sexual orientation, childcare arrangements, or other family plans. Federal and State laws such as The Americans With Disabilities Act (ADA,) the Pregnancy Discrimination Act, and the Civil Rights Act of 1964 (Title VII,) which was amended in 1991, allow you to avoid discussing any of this information. The interviewers ask should focus exclusively on the position and your experience and qualifications for it. You do not have to reveal any private information that does not relate to your ability to perform on the job.
Your interviewer might not realize they're asking an illegal question - they might think they are simply breaking the ice. So if they do ask a question of this sort, you should make a decision about their intentions and whether or not you want to answer it. In some cases, the question might be innocuous enough that you'll feel comfortable answering it. If you don't wish to discuss something that you fear might wrongfully be used against you, then you should tactfully say that question does not relate to your abilities to perform the job, abilities that include x,y, and z.
Here are some examples of illegal questions:
Are you married?
Do you have any kids? Are you planning to have any(more)?
What political party do you belong to?
Do you have any illnesses we should know about?
What nationality are you?
What church do you go to?
Do you smoke?