Microorganisms can be classified into different types based on their size, shape, reproduction method, nutrition, and habitat. The main types include bacteria, viruses, fungi, protozoa, and algae. Bacteria are unicellular microbes that reproduce through binary fission, while viruses are the smallest and can only replicate inside living host cells. Fungi reproduce both sexually and asexually, and protozoa include both parasitic and saprophytic single-celled organisms. Algae are similar to plants but lack true roots, stems, and leaves.