There are 7 main taxonomic categories. They are obligate categories i.e., they are strictly used at the time of any plant classification.
Biggest group ->  Kingdom
->  Phylum in animals / Division in plants
->  Class
->  Order-Cohort
->  Family
->  Genus
->  Species 

There are some extra categories, like sub division, sub order, sub family, tribe, sub tribe,. etc. They are not regularly used. They are used only when they are needed

• The sequence of arrangement of taxonomic categories in a descending order during the classification of an organisms is called taxonomic hierarchy.
• Kingdom is the highest and species is the lowest category in this hierarchy.
• Plant groups or animal groups included in categories are called taxon.
• Suffix for taxa (Taxon)

Division – phyta

Sub division – phytina

Class – opsida, phyceae, ae

Order – ales

Sub-order – ineae

Family – aceae

Sub family – oideae

Tribe – eae

Sub tribe – inae

Notes: There is no suffix for Genus, Species and Kingdom.

SPECIES

Species is the smallest taxonomic category. It is the basic unit of classification.

John Ray proposed the term and concept of species (1942).

BIOLOGICAL CONCEPT OF SPECIES

• Mayr proposed the biological concept of species.
• According to Mayr "all the members that can interbreed among themselves and can produce fertile offsprings are the members of same species"
• But this definition of Mayr was incomplete because this definition is applicable to sexually reproducing living beings. There are many organisms that have only asexual mode of reproduction.

E.g., Bacteria, Mycoplasma.

• The main character in determination of any species is interbreeding. But this character is not used in taxonomy. In taxonomy, the determination of species is based on other characters.

E.g., mainly morphological characters.

STATIC CONCEPT OF SPECIES

• The static concept of species was proposed by Linnaeus.
• According to Linnaeus "species is unchangeable" i.e. there is no change in the character of species. The species of present day are same as they were in past and they will remains same in future.

DYNAMIC CONCEPT OF SPECIES

• This concept was proposed by "Lamarck".
• According to this concept, "species is always changeable". Changes always occur in the characters of species from one generation to next generation. And these changes are known as "evolution".

TYPOLOGICAL CONCEPT

• This concept was proposed by "Aristotle" and "Plato".
• According to this concept, "there is a definite type or pattern of characters in each species of every living organisms and all the members of species show maximum resemblance with this pattern". (Typological concept is based on single individual of species).

Biotype : Members of same species inhabiting similar environment and having some genetic variations are known as biotypes. Variations found in these members are permanent. These members cannot interbreed among themselves.

E.g., Cauliflower, cabbage, knol-khol are three biotypes of one species.

Ecotypes  : Members of same species inhabiting different environment and having some genetic variations are known as ecotypes. Variations are permanent. These members can interbreed among themselves but due to geographical barrier they cannot interbreed.
E.g., Crow (Corvus splendens) found in different regions are ecotype of one species.

Ecads or Ecophenes : Members of same species having some non genetic variations due to environment is called Ecads. These variations are temporary.

Definition related to species

Allopatric species : Those species that are found in different geographical regions and have geographical barriers between them are known as allopatric species. Geographical barriers are hills, oceans, himalayan mountains.

Sympatric species : The species found in similar geographical regions are sympatric species.

GENUS

• Genus is an assembly of related species which involved from a common ancestor and have certain common characters called correlated characters.
• Potato, tomato and brinjal are three different species but all belong to the genus Solanum. Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) with several common features, are all species of the genus Panthera. This genus differs from another genus Felis which includes cats.

FAMILY

• Family, has a group of related genera with still less number of similarities as compared to genus and species.
• Families are characterized on the basis of both vegetative and reproductive features of plant species.
• Three different genera Solanum, Petunia and Datura are placed in the family Solanaceae. Among animals for example, genus Panthera, comprising lion, tiger, leopard is put along with genus, Felis (cats) in the family Felidae.

ORDER

• Order being a higher category, is the assemblage of families which exhibit a few similar characters. The similar characters are less in number as compared to different genera included in a family.
• Plant families like Convolvulaceae, solanaceae are included in the order polemoniales mainly based on the floral characters.
• The animal order, Carnivora, includes families like Felidae and Cancidae.

CLASS

• A class is a subdivision within a phylum made of one or more related orders.
• Order Primata comprising monkey, gorilla and gibbon is placed in class Mammalia along with order Carnivora that includes animals like tiger, cat and dog. Class Mammalia has other orders also.

PHYLUM

Classes comprising animals like fishes, amphibians, reptiles, birds along with mammals constitute the next higher category called phylum. All these based on the common features like presence of notochord and dorsal hollow neural system, are included in phylum Chordata. In case of plants, classes with a few similar characters are assigned to a higher category called Division.

KINGDOM

All animals belonging to various phyla are assigned to the highest category called Kingdom Animalia. The Kingdom Plantae, on the other hand, is distinct, and comprises all plants from various divisions.