An open question is one that has many variables in the type of answer that can be given. A closed question anticipates a specific answer. You will use both types of questions during your interviews. Open questions allow the interviewee to do most of the talking and that is after all what you hope to achieve with your questions. Closed questions are necessary to get information that is essential to place the interview in a historical context or to find out more about an answer to an open question.
Open questions provide a lot of useful, as well as useless, information.
Try and develop some open questions beforehand. Closed questions are easier to think of before and during an interview. The clearer the question the clearer the answer is likely to be. By making the question too difficult or too long you run the risk of confusing your interviewee. Do not ask two or more questions at the same time, as you will probably only get an answer to one.