Ans. Microorganisms are tiny living organisms that can only be seen under a microscope. They are classified into four main groups: bacteria, fungi, protozoa, and viruses. Bacteria are single-celled organisms that can exist in various shapes and sizes. Fungi are multicellular organisms that include molds and yeasts. Protozoa are unicellular organisms that are often found in water bodies. Viruses are non-living particles that can only replicate inside the cells of other living organisms.