An organism is an individual, living entity. Organisms range in size and complexity from tiny microorganisms, to large, multicellular plants and animals. Biologists classify organisms into five kingdoms on the basis of their cellular and sub-cellular organization, metabolism, reproduction, and behavior. These kingdoms are described below, in order of their earliest appearance in the fossil record of life.

Monera are prokaryotic microorganisms, meaning their genetic material is not contained within a bounded organelle called a nucleus. This group includes the simplest organisms, including viruses, which consist of little more than a protein shell containing nucleic acids. Viruses are incapable of reproduction unless they parasitize the metabolism of an unrelated host cell. Blue-green bacteria and true bacteria are two other groups of monerans.

Protista are a diverse group of microorganisms, including the simplest eukaryotic organisms, which have an organized nucleus, one or more flagellae, and usually mitochondria and plastids...