Questions are a good opportunity for you to interact with your audience. It may be helpful for you to try to predict what questions will be asked so that you can prepare your response in advance. You may wish to accept questions at any time during your presentation, or to keep a time for questions after your presentation. Normally, it's your decision, and you should make it clear during the introduction. Be polite with all questioners, even if they ask difficult questions. They are showing interest in what you have to say and they deserve attention. Sometimes you can reformulate a question. Or answer the question with another question. Or even ask for comment from the rest of the audience.