In the context of marketing research, descriptive research is research that attempts to answer very specific questions about the relationships between certain variables. It can be differentiated from exploratory research.
Exploratory research occurs when a researcher really does not know much about the subject that they need to research. At that point in the process, they do not have any hypotheses about why a certain condition is occurring. They do exploratory research to try to formulate hypotheses that might help to explain the condition.
That is when descriptive research comes into play. Once the researcher has a hypothesis, they test it through descriptive research. For example, a researcher might hypothesize that sales are declining because people in a certain demographic category are not responding well to a certain change in the product. The researcher could conduct descriptive research on this subject. They could try to determine whether the people in that demographic category had changed their behaviors. This is a much more specific and focused sort of research than exploratory research.