A questionnaire is a research tool used to collect information from respondents through a series of questions. It has both advantages, such as being easy to construct and distribute, and disadvantages, like some respondents not providing accurate or complete answers. Properly constructing a questionnaire involves steps like researching similar existing questionnaires, writing initial questions, editing the questions, pretesting the questionnaire, and ensuring questions are clear, unbiased, and will provide relevant data for the research.