When conducting marketing research, it is possible to utilize primary data and secondary data. These two differ in how and when they have been collected. They are useful for different things.
Primary data is data that you collect yourself in the process of conducting your study. This can be things like data from customer surveys. It can be data that you have amassed while watching customers shopping in your store and interacting with your sales staff. By contrast, secondary data is not data that you have collected. It is data that is already present before you begin your research. An example of this would be government census data for the area in which your store is located.
Each of these types of data has its own advantages and disadvantages. Secondary data has already been collected. You do not need to go to the bother or expense of obtaining it yourself. Secondary data can give you more information than you could possibly compile on your own. It might tell you about the ethnic and economic makeup of your customer base, which is something you could not have determined on your own.
On the other hand, primary data can be much more tailored to your own needs. The secondary data has not been collected with your needs in mind. Primary data can be because you are setting up the research that obtains that data. Primary data, then, is typically more fine-grained and useful to your specific needs, even if it is harder to obtain.