Living organisms show certain key characteristics which distinguish them from non-living things. These are
- Growth is shown by living organism by an increase in mass and an increase in the number of individuals. A multicellular organism grows by cell division.
- Reproduction is the process of producing offspring possessing features similar to those of their parents. It takes place by sexual or asexual mode.
- Metabolism comprises of both constructive reactions (anabolism) and destructive reactions (catabolism), continuously occurring in the body.
- Cellular organisation The cells are the building blocks of all living organisms may it be plants, animals or humans. Thus, organisms can be unicellular or multicellular.
- Consciousness is the ability of living organisms to sense their surroundings or environment and respond to these environmental stimuli, which could be physical, chemical and biological.

- Biodiversity refers to the number and types of organisms present on earth. Our earth possesses a wide range of living organisms. A number of plants and animals have been identified and described. However, a large number of organisms are still unknown to us.
- A rich diversity among organisms or biodiversity in terms of size, colour, habitat, physiological and morphological features can be observed on earth. Therefore, it is necessary to standardise the methods to identify and classify them on the basis of their defining characteristics.

- Certain rules and principles have been formulated for the identification, nomenclature and classification of organisms, which facilitate the study of vast diversity of organisms present on earth.
- Identification involves the process of finding the correct name and place of an organism. The morphological and anatomical characters are examined for proper identification.
- Nomenclature involves standardising appropriate naming of living organisms, so that they can be recognised and differentiated from others easily across the world.
- To ease the process of studying different organisms, a scientific name is assigned to each organism.
The principles of naming have been established by International Code for Botanical Nomenclature (ICBN) and International Code for Zoological Nomenclature (ICZN) for plants and animals, respectively.
- Organisms are identified on the basis of their resemblance and distinct differences from others. They are assigned a correct scientific/biological name.
- Binomial System of Nomenclature was developed by Carolus Linnaeus in 1751 and was published in his book Species Plantanum (1753). As per this system, a biological name comprises of two words namely, generic name and the specific epithet.
- Nomenclature of organisms follows certain universal rules, which are as follows

- The system of arranging different categories or ranks, which are referred to as taxonomic categories in a proper ascending or descending order is called as taxonomic hierarchy. Every organism occupies a distinct position in a taxonomic hierarchy.
- Each category in taxonomical hierarchy is commonly called taxon. The term 'taxon' was first introduced by ICBN during 1956 and it is the basic unit of classification.
- Taxonomic hierarchy was first proposed by Linnaeus and thus it is also called as Linnaeus hierarchy. This hierarchy constitutes the following components in an ascending order.
- Species It is the smallest unit of taxonomic hierarchy consisting of groups of morphologically similar individuals which can interbreed to produce offspring, e.g. nigrum and melongena are the two species of genus-Solanum.
- Genus It comprises of a group of related species having more characters in common in comparison to species of other genera, e.g. lion, leopard and tiger are all species of the genus-Panthera, while cats belong to the genus-Felis.
- Family It is a group of related genera with a few common features but less number of similarities as compared to genus and species. Plant families are categorised on the basis of both vegetative and reproductive features of species, e.g. family-Solanaceae possesses different genera like Solanum, Petunia and Datura. Similarly, in animals, cats and dogs belong to two different families-Felidae and Canidae, respectively.

Biologists have established certain procedures and techniques to store and preserve information as well as the specimens which are useful in identification and classification of organisms. These techniques stored information and procedures are called taxonomic tools or taxonomic aids. Following are some of the main taxonomical aids used to study taxonomy
- Herbarium is the storehouse of collected plant specimens that are dried, pressed and presserved on herbarium sheets. The biggest herbarium of the world is the Royal Botanical Garden in Kew (England), while the biggest herbarium of India is the Central National Herbarium at Shibpur (Kolkata).
- Botanical garden is essentially a collection of living plants maintained for both pure and applied studies. The famous botanical gardens are Royal Botanical Garden in Kew (England), Indian Botanical Garden, Howrah (India) and National Botanical Research Institute, Lucknow (India).
- Museums have a collection of preserved plants and animals for study and reference purposes. Specimens are preserved in containers or jars in preservative solutions. Insects are preserved in insect boxes after collecting, killing and pinning them. Larger animals like birds and mammals are usually stuffed and preserved.
- Zoological parks are the places where wild animals are kept in protected environments under human care. This enables us to learn about their food habits and behaviour.