Zoology or animal biology, is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification,habits, and distribution of all animals, both living and extinct. The term is derived from Ancient Greek zōon - animal and λόγος - knowledge, study.


Cell biology studies the structural and physiological properties of cells, including their behavior, interactions, and environment. This is done on both the microscopic and molecular levels, for single-celled organisms such as bacteria as well as the specialized cells in multicellular organisms such as humans. Understanding the structure and function of cells is fundamental to all of the biological sciences. The similarities and differences between cell types are particularly relevant to molecular biology.Anatomy considers the forms of macroscopic structures such as organs and organ systems.


Scientific classification in zoology, is a method by which zoologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy. Modern biological classification has its root in the work of Carolus Linnaeus, who grouped species according to shared physical characteristics. These groupings have since been revised to improve consistency with the Darwinian principle of common descent. Molecular phylogenetics, which uses DNA sequences as data, has driven many recent revisions and is likely to continue to do so. Biological classification belongs to the science of zoological systematics.

Many scientists now consider the five-kingdom system outdated. Modern alternative classification systems generally start with the three-domain system: Archaea; Bacteria; Eukaryota (including protists, fungi, plants, and animals). These domains reflect whether the cells have nuclei or not, as well as differences in the chemical composition of the cell exteriors.
Further, each kingdom is broken down recursively until each species is separately classified. The order is: Domain; Kingdom; Phylum; Class; Order; Family; Genus;Species. The scientific name of an organism is generated from its genus and species. For example, humans are listed as Homo sapiens. Homo is the genus, andsapiens the species. When writing the scientific name of an organism, it is proper to capitalize the first letter in the genus and put all of the species in lowercase. Additionally, the entire term may be italicized or underlined.
The dominant classification system is called the Linnaean taxonomy. It includes ranks and binomial nomenclature. The classification, taxonomy, and nomenclature of zoological organisms is administered by the International Code of Zoological Nomenclature, and International Code of Nomenclature of Bacteria for animals and bacteria, respectively. The classification of viruses, viroids, prions, and all other sub-viral agents that demonstrate biological characteristics is conducted by theInternational Code of Virus classification and nomenclature. However, several other viral classification systems do exist.