Taxonomy is the branch of biology dealing with the identification, nomenclature and classification of organisms based on their characteristics. A taxon is a unit or group of organisms of any rank in classification (for example, mammals, insects, dogs). Taxonomic hierarchy is the arrangement of taxonomic categories in a definite descending order, from kingdom down to species.

Binomial nomenclature is the system of naming organisms with two words, given by Carolus Linnaeus, where the first word is the genus and the second is the species (for example, Mangifera indica for mango). Rules: (1) the generic name begins with a capital letter and the specific name with a small letter; (2) the biological name is printed in italics (or underlined separately when handwritten) and is usually in Latin.

Kingdom Monera includes bacteria, which are prokaryotic (they lack a true membrane-bound nucleus and membrane-bound organelles). They are microscopic, unicellular organisms with a cell wall (except mycoplasma). Their nutrition may be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic or parasitic). They are grouped as Archaebacteria (live in extreme habitats) and Eubacteria (true bacteria).