In marketing research, there are two sorts of data that can be used. These types of data are primary data and secondary data. The difference between the two is in when, by whom, and for what purpose these are compiled.
Secondary data is data that has been compiled before a given research project begins. The data in question was not typically compiled by the people doing the marketing research. Just as importantly, it was not acquired for the specific purpose that the research project is trying to accomplish.
Secondary data, then, is not as useful as primary data, which is compiled for a specific purpose. Secondary data cannot typically answer the specific questions being asked in the research project. For example, imagine that you want to know what it is that people like about a given product that you sell. If you have secondary data that shows how many products like yours people in your area tend to buy, it will not be very useful for your specific purpose.
Secondary data is data that is already in existence before a research project begins and which was not compiled for the purposes of that project.