Biologists classify organisms based on physical characteristics and genetic relationships. Each specific kind of living thing is given a designated species name. Groups of related species are grouped together into a genus. Related genera are grouped into families, and families that show close relationships are grouped into orders. Orders are grouped into classes, classes into phyla or divisions, and phyla into kingdoms.
Kingdoms are the broadest level of classification of living creatures. Most biologists agree that organisms are divisible into six kingdoms, though earlier versions of the classification system rely on only five. The six kingdoms in the modern system are: Monera, which contains unicellular bacteria that lack nucleii, Archaea, which contains unicellular bacteria that feature nucleii, Protista, which contains unicellular organisms with plant-like or animal-like characteristics, Fungi, which contains multicellular organisms which absorb nutrients from an external source, Plantae, which consists of multicellular organisms that are capable of photosynthesis, and Animalia, which consists of multicellular organisms that obtain and break down complex nutrient molecules internally.
Each Kingdom is subdivided into Phyla, each Phylum into several classes, and so forth. The further down the classifications scheme, the more similar its members. Common Phyla in the Kingdom Animalia, for example, are Molluska, which contains shelled mollusks that live in water, and Nematoda, which contains several classes of thread-like roundworms.