What Not to Ask
Avoid questions that give the wrong impression. If you ask about things like vacations or personal days, you're indicating that you're more concerned with taking time off than with doing your job. Don't discuss benefits or perks at this stage -- that will be part of the salary discussion if you're offered the job. Don't inquire about other candidates the company is considering for the job, either. Keep the focus on yourself.
Asking the right question during an interview is almost as important as giving good answers. The questions you ask should show your interest in the job. They can demonstrate that you've taken the time to learn something about the company and the position. Not asking good questions suggests a lack of interest.
For example, you might ask about the challenges of the job. What are likely to be the main projects you'll be working on? What skills will you need in order to advance? What positions might come open in the future? One of the important things the interviewer is looking for is your commitment to the job. No company wants to hire and train an employee who will soon move on.
Ask for details related to the job, too. To whom will I be reporting? Who are the other members of the team I'll be working with? How much travel will be involved? What are main goals of the department where I'll be working?