The term Biodiversity or Biological diversity means the number and types of organisms present on the earth, forms of life in the living world. The living world includes all the living organisms, such as microorganisms, plants, animals and humans. Biodiversity is not limited to the existing life forms. This chapter elaborates fundamental concepts such as “What is Living?” Diversity in the Living World, Taxonomic Categories, Taxonomical Aids and more. The living world is rich in variety. Millions of plant and animal species have been identified and described till date, but countless more remain yet to be discovered. The taxonomic studies of various species of plants and animals are useful in agriculture, forestry, industry and in general for knowing our resources and their diversity. Taxonomists have developed a variety of taxonomic aids to facilitate identification, naming and classification of organisms. The extraordinary habitats in which living entities are found can be extremely cold conditions or extremely hot conditions, deciduous forests, freshwater lakes, oceans, hot springs and so on.