A selective medium is a substance (usually agar based) which grows a specific type of microbe. Using different nutrient ratios and incubation times speciation of bacterium can be done without use of restriction enzymes of powerful microscopes.
Good examples of differential media are: YM (Yeast and Mold) which has a low pH, deterring bacterial growth. Blood agar (used in strep tests), which contains beef heart blood which becomes clear in the presence of hemolytic streptococus.