What is this?
There are many ways to categorize questions, but two of the most important classifications to use in persuading others is open-ended and closed-ended.
Why is this important?
These two types of questions are basically two very different types of tools used in persuading others and each are ideal for different aspects of selling. Open-ended questions leave an open range of options for answering the question and usually are good for gathering large amounts of data with which we can later work. Closed-ended questions close off the range of answers to "yes" and "no" athough we often get "maybe".
In the very early stages of persuasion it can be useful to use to almost rhetorical (another way of classifying questions) closed-ended questions in a short succession to elicit "yes" answers to help in the development of rapport. As the other person opens up to you closed-ended questions will be useful to explore their situation and whether they are a prospect for what you have to offer.
When you have determined that your product or service can genuinely benefit your prospect then the process of closing the sale (coming to agreement or however you prefer to refer to the completion of the exchange) will usually require you to possess at least some skill in using closed-ended questions. People who try to come to agreement with others using open-ended questions are often frustrated and unsuccessful in gaining their commitment.