Classification: The process by which anything is grouped into convenient categories based on easily observable characters.
Taxa (singular: taxon): The scientific term for these categories. Taxa can represent categories at different levels (e.g., dogs, mammals, and animals are all taxa at different hierarchical levels).
Taxonomy: The process of classification of all living organisms into different taxa based on their characteristics.
The basis of modern taxonomy includes external and internal structure, cell structure, development process, and ecological information.
The processes basic to taxonomy are characterisation, identification, classification, and nomenclature.
Systematics: A branch of study that deals with the different kinds of organisms, their diversities, and the relationships among them.
Derived from the Latin word ‘systema’, meaning systematic arrangement.
Linnaeus used Systema Naturae as the title of his publication.
Modern systematics includes identification, nomenclature, classification, and crucially, evolutionary relationships between organisms.