There are two basic types of data and they are qualitative and quantitative. Qualitative data is data that is often put into categories not based on numbers but often some other form of commonality. For example, if a person conduct interviews about student satisfaction, certain concepts, such as good teaching, may be repeated several times by different students. These statements are combined into one category of student satisfaction, which would be good teaching. There is no continuum of data in qualitative it is strictly the development of categories based on a criteria developed by the researcher.
Quantitative data is numerical data that is often based on a continuum. Example of quantitative data is such things as height, weight, and age. You can treat quantitative data like qualitative by developing categories but this is a discussion for the future.
When to collect qualitative and quantitative data depends on the research questions of the researcher. Neither is superior to the other and it is the context that determines what is best.