Questionnaires are used to collect data through a set of questions asked to target respondents. There are different types of questionnaires including structured, unstructured, and combination questionnaires. Questionnaires contain different types of questions such as open-ended questions, closed-ended questions, rating questions, and Likert scale questions. Questionnaires can be conducted through various methods like mailed surveys, e-mailed surveys, telephone surveys, personal administration, and personal interviews. Each method has its own advantages and disadvantages related to factors like response rate, costs, and standardization.