In the laboratory, microorganisms grow on or in different kinds of culture medium. These contain everything microorganisms needs to live, e.g. water, carbon, nitrogen, phosphate, sugar, peptones, salts and so on. There are many different kinds of culture medium, depending on the type and lifestyle of the organisms, but the main difference is between solid and liquid culture medium.
Explanation for children: To prevent microorganisms from starving in the laboratory, they are grown on or in culture medium. These are either liquid or solid and contain important nutrients for the microorganisms.