First of all, please note that different books might give different “main types” of descriptive research methods. Please check your own text to make sure that you have the right answer.
One possible answer is to say that the three main types of methods are survey methods, observational methods, and case study methods
Survey research is done by asking a large number of people a given set of questions. For example, marketing researchers might contact people by telephone and ask them if they have seen a certain advertising campaign. They might try to determine what percentage of people have seen and remembered the ads and what impressions the ads had on those people.
Observational methods might be conducted in a store. A marketer might watch people as they shop, gathering data on how long they spend in certain parts of the store, what they look at, and where they actually make purchases.
Case study methods are somewhat less important for this sort of research. These consist of making an in-depth study of a few individuals to try to gain a deeper understanding of things like their motivations when making purchases.