There are diverse habitats on earth hosting millions of living organisms. Due to limited number of taxonomists, absence of thorough survey of different areas and occurrence of several inaccessible regions, e.g., hot springs, underwater reefs, etc., it is difficult to identify all the organisms.

Living organisms can be divided into different taxa based on their characteristics.This process of classification is called taxonomy.

Few nomenclature Codes are
(i) International Code of Botanical Nomenclature (ICBN)
(ii) International Code of Zoological Nomenclature (ICZN)
(iii) International Code of Viral Nomenclature (ICVN)
(iv) International Code of Bacteriological Nomenclature (ICBN)

The basic processes in taxonomy are
(i) characterisation
(ii) identification
(iii) classification
(iv) nomenclature

Scope of systematics is
(i) Identification of organisms
(ii) nomenclature of existing and new organisms
(iii) classification of new organisms
(iv) concluding evolutionary relationships between different organisms
Systema Naturae is the publication of Linnaeus.

We should call it biodiversity or biological diversity as it shows variety of living organisms present on earth.

There are many organisms, which never reproduce in their life, although all other characteristic of living things are present in them, e.g., sterile worker bees, mules, infertile human couples, etc., Hence, reproduction cannot be an all-inclusive defining property of living things.

The number and kinds of organisms are never constant at a place, as development and reproduction keep the number changing. This is influenced by season and availability of resources, e.g., number of mosquitoes is very large in warm and humid weather, whereas in extreme summer or winter, they can be seen rarely.

The organisation in a  living body produces a new functional entity, which has its own properties not found in its components. Hence, a cell organelle is the one having its own properties not found in its molecular constituents.

All organisms from the prokaryotes to the most complex eukaryotes can sense and respond to their environment.
The human have five sense organs, which help them to sense their environment.These are eyes, ears, nose, tongue and skin.

The sum total of all chemical reactions occurring in an organism due to specific interactions amongst different types of molecules within the cells, is called metabolism.

In unicellular organisms like Amoeba, growth is synonyms with reproduction and takes place by increase in number of cells by mitotic cell division.

In unicellular organisms like Amoeba, bacteria and unicellular algae, reproduction is synchronised with growth, i.e. they reproduce and grow by increase in number of cells.

Metabolic reactions can be demonstrated outside the body in cell-free systems. When an isolated metabolic reaction is performed outside the body of an organism in a test tube, it is neither living nor non-living. Hence, while metabolism is a defining feature of all living organisms without exception, isolated metabolic reactions in vitro are not living things, but are living reactions. Here, it can also be said that cellular organisation of the body is the defining feature of life forms.

All the activities like growth, reproduction, movements, responsiveness, development, etc., are due to metabolism.
Metabolism is of two types, i.e., anabolism and catabolism. Anabolism involves the synthesis of complex compounds from simpler ones, e.g., photosynthesis and catabolism involves the breakdown of complex compounds into simpler substances, e.g., respiration.

Taxonomical aids are helpful in various fields such as agriculture, forestry, industry and in knowing our bioresources.
All the above-mentioned studies need correct identification and classification of organisms. Identification of organisms requires intensive laboratory and field studies.
Biologists use herbarium, botanical gardens, museum, zoological parks, and taxonomical keys while doing taxonomical studies.

(i) The taxonomic categories which are always used in hierarchical classification of organisms are called obligate categories.
(ii) The taxonomic categories from species to kingdom are broad categories or obligate categories. However, taxonomists have developed sub-categories in this hierarchy to facilitate more sound and scientific placement of various taxa.
(iii) These are intermediate categories.
(iv) They include subphyla, subclasses, suborders, subfamilies, subgenera, varieties or subspecies.

Each category in taxonomical hierarchy is commonly called taxon.
The different taxonomical categories are-species, genus, family, order, class, phyla/division and kingdom. Some examples of taxa at different hierarchical levels are
(i)  Mangifera indica is a taxon and species is its category.
(ii)  The taxon of all the bacteria is Monera and the category is kingdom.
(iii) The taxon of fishes is Pisces and the category is class.