There are about 1.2 million animal species which show a great diversity of life.
Animals show a great diversity in their heterotrophic mode of obtaining food.
According to their mode of living, animals may be solitary (found one e.g. Tapeworm).
They can be gregarious (live in groups but without obligatory interactions) or colonial (live in social hive and show obligatory interdependence e.g, honeybee wasps ).
So to express the relationship between different organisms, there is need of classification.
It involves grouping on the basis of their similarities and differences.