a question that has no known solution up to this moment, and it is not known wheter the problem has one or not
a question that requires a sentence to answer, rather than just a single word
a question that requires thought and the other person to communicate with you in a sentance
A question that encourages the customer to provide a lot of information (see closed question), for example "Can you please describe the problem?".
a question that gains information, usually beginning with who, what, why, where, when, how, or more subtly 'tell me about..'
An open-ended question designed to promote higher-order, independent thought. Responses are expected to be reflective and require thinking time. Questions like this are also known as 'fat' or 'higher-order' questions.
A question which does not have a range of pre-specified responses, i.e. respondents can answer as they wish.