Species is the specific name given to a living organism. When using binomial nomenclature to give a scientific name to an organism, the second name in the two-part naming system is always the species of that organism. The scientific name for humans is Homo sapiens. All organisms in a particular species are basically the same in every major way. The main criteria for an organism to be admitted to a particular species it the ability to interbreed. All organisms of a particular species can reproduce with each other.
The species is the lowest and most specific level of the classification of living things system. Organisms of a species may have varying appearances but they are the same when it comes to number of cells, functionality, abilities, and reproduction. Once organisms are filed into a genus, they are then further divided into a species. No two different species are exactly the same. They may appear the same because they are in the same genus, but there must be a difference about them that puts them in a different species.
Some genera appear to have many more species than others. This is only because certain animals are studied extensively; therefore more discoveries are being made for those groups of animals. There are millions of undiscovered species on Earth. Some animals are so small that they are hard to discover. It is estimated that there are up to 30 million different species of animals on Earth.