Microorganisms, plants and animals as well as their cell cultures are used in industrial microbiology and biotechnology. However, microorganisms have some advantages over plants or animals, as inputs in industrial microbiology and biotechnology. Microorganisms grow rapidly in comparison with plants and animals. Biotechnological products which can be obtained from microorganisms in a matter of days may take many months in animals or plants. The space requirement for growth of microorganisms is small. A 100,000 liter fermentor can be housed in about 100 square yards of space, whereas the plants or animals needed to generate the equivalent of products in the 100,000 liter fermentor would require many acres of land. Microorganisms are not subject to the problems of the changes of weather which may affect agricultural production especially among plants. This chapter presents nature of cells of living systems, structure and function of prokaryotic and eukaryotic cell types, differences among the three domains of life (bacteria, archaea and eukarya), taxonomic grouping of industrial microorganisms with emphasis on the proteobacteria, the firmicutes and the actinobacteria, and the characteristics of acetic and lactic acid bacteria. The chapter embodies essentials of microbiology for non-microbiologists who study industrial microbiology; and reinforces general microbiology knowledge.