Kingdoms are the major categories into which scientists divide up all living things. The main kingdoms are animals, plants, fungi and bacteria, although there are others. Each kingdom has its own suite of defining characteristics - for instance plants have rigid cell walls, whilst animals do not.
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories. The rank of species, and specification of the genus to which the species belongs is basic, which means that it may not be necessary to specify ranks other than these.