Microorganisms were the first in the evolution of life on earth 3–4 billion years ago. It had been a mystery till the discovery of microorganisms as to why grapes could be turned into wine, milk into cheese or why food would spoil. Bacteria, archaea, fungi, algae, viruses and most of the protists are almost always microscopic and are considered as microorganisms.

Prokaryotes are organisms which are mostly unicellular and lack a cell nucleus. Organelles are found in eukaryotes and they are divided into two groups, the archaea and the bacteria. Bacteria are ubiquitous and there are approximately five million bacteria in the world forming much of the world’s biomass. Bacteria are vital in recycling nutrients such as the fixation of nitrogen from the atmosphere. There are approximately ten times as many bacterial cells as human cells in the human body and most of these bacteria are rendered harmless or beneficial by the protective effects of the immune system. However, a few pathogenic bacteria cause infectious diseases, the most common being respiratory infection.

In industry, bacteria are important in waste water treatment, production of cheese, yoghurt and antibiotics.

The fungi are heterotrophic organisms, living for the most part in soil, dead matter and as symbionts of plants, animals or other fungi. They perform an essential role in all ecosystems in decomposing matter and are indispensable in nutrient cycling and exchange. Many fungal species have long been used as a direct source of food such as mushrooms and in fermentation of various food products such as wine, beer and soy sauce.

A virus (from the Latin virus meaning toxin or poison) is a sub–microscopic infectious agent that do not have cells and is unable to grow or reproduce outside a host cell. Each viral particle or virion consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. They have been described as organisms at the edge of life. Viral infections in human and animal hosts usually result in an immune response and disease. Often, a virus is completely eliminated by the immune system. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life–threatening infections. Vaccines that produce lifelong immunity can prevent virus infections such as polio, measles, mumps and rubella. Antiviral drugs are being developed to fight diseases like HIV and Hepatitis B virus infections.

Algae are a large and diverse group of simple plant–like organisms ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds which are an important source of food rich in iodine, potassium, iron, magnesium and calcium along with many vitamins including: A, B1, B2, B6, niacin and C. Algae are considered "plant–like" because of their photosynthetic ability but they lack the distinct organs such as leaves and roots that characterize higher plants. Algae play significant role in aquatic ecology as microscopic forms that live suspended in the water column – called phytoplankton – provide the food base for most marine food chains. Algae are used as fertilizers, soil conditioners, live stock feed, biomass and make vegetable oil and biodiesel.

Protozoa (in Greek proto = first and zoa = animals) are one–celled eukaryotes which play a vital role in controlling bacteria population and biomass. Protozoa decompose organic matter and thereby help in converting organic wastes into simpler useful soil nutrients. Protozoans form an important link in aquatic food chains as they feed on algae and become food for other protists which in turn become food for others.