Towards the end of the interview, the interviewer is likely to ask if you have any questions. This is your opportunity to find out more about the job. The interview is a two-way process - you want to make sure you have enough information to decide if you want the job should it be offered.
Here are some examples of questions to ask if they have not already been answered in the interview or covered in the organisation's recruitment literature or web site:
Just ask one or two questions, whatever is most important to you.
What if they've answered all your questions?
If the interviewers have answered all your questions, say so and use this as an opportunity to tell them something you want them to know about you that didn't come up in the interview, or reiterate your interest in the position and the company.
When the interviewer asks "Do you have any questions for us?" it indicates that the interview is almost over, and it is your opportunity to end it on a positive note.