In the hierarchical classification of living things, "kingdom" comes second in the taxonomic ranks, right below "domain."

And, in biology, all living organisms are divided into prokaryotes and eukaryotes. Prokaryotes belong to the domains Archaea (Archaebacteria) and Bacteria (Eubacteria). Meanwhile, eukaryotes belong to the domain Eukarya.

All prokaryotes are unicellular, but eukaryotes are usually multicellular. In addition, eukaryotes have a membrane-bound nucleus, and their DNA is contained in that nucleus. Meanwhile, prokaryotes don't contain a membrane-bound nucleus. So, its DNA is instead found in the nucleoid, which floats in the cytoplasm.

Within the domain Eukarya, there are five kingdoms. They are Animalia, Plantae, Protozoa, Fungi, and Chromista.

Specifically, the kingdom Chromista was founded by the English biologist Thomas Cavalier-Smith in 1981 to differentiate between some protists and protozoans. However, in 2009, Cavalier-Smith maintained that the kingdom Chromista was actually derived from the kingdom Protozoa. He asserted that both kingdom Chromista and Plantae were sister kingdoms.

Essentially, there are seven kingdoms in all, according to Cavalier-Smith's classification system: Archaea, Bacteria, Plantae, Animalia, Protozoa, Fungi, and Chromista.

Chromista includes photosynthetic chromists that contain chlorophyll c, chlorophyll a, and fucoxanthin. It also includes all protists, marine algae, and oomycetes. The latter are fungus-like eukaryotic organisms that can cause many dangerous plant diseases. In all, Chromista consists of organisms that contain either or both plastids and cilia.