Life is the most important concept in biology. The word itself refers to a living being or to its processes. It may mean the time a being is alive.
Kingdom is the highest level of classification, which is divided into subgroups at various levels. There are five kingdoms in which the living organisms are classified, namely, Animalia, Plantae, Fungi, Protista, and Monera.
Class was the most general rank in the taxonomic hierarchy until phyla were introduced. Kingdom Animalia includes 108 classes including class Mammalia, Reptilia, Aves, etc. However, the classes used today are different from those proposed by Linnaeus and are not used frequently. For example, when it was discovered that birds are descendants of earlier dinosaurs, that caused some difficulties. In fact they are now so different from the original dinosaurs that it makes sense to teach and refer to them separately from dinosaurs.
A genus is a group of similar species. Some genera have only one species and are known as monotypic, whereas others have more than one species and are known as polytypic. For example, lion and tiger are placed under the genus Panthera
Species is the lowest level of taxonomic hierarchy. There are about 8.7 million different species on Earth. The word refers to a group of organisms that are similar in shape, form, reproductive features. Species can be further divided into sub-species.