Ideally, you will have found out as much about the job as possible before you decide to apply.
However, if you get to the interview stage, you will probably still have questions you want to ask the selection panel.
As a general rule, don't ask too many questions and make sure that they impart your positive attitude towards the job.
Be careful how you ask questions about salary, promotion, career development and the like. Asking these types of question may give the impression that you are only interested in how the job will benefit you, rather than what you can offer. If possible, phrase your questions so that you stress the benefits to the section rather than to yourself.