Binomial Nomenclature

Once Linnaeus had decided on a basis for classifying organisms, he then developed a system for naming them. His system is quite simple. He gave each species a name that consists of two words. This system is called binomial nomenclature. He used Latin words for these names because all scientists wrote in Latin in time of Linnaeus. Thus, the human is Homo sapiens, and the domestic (house) cat is Felis domesticus. The first word of each name is called the genus and the second word is called the species. The genus begins with a capital letter and the species does not. The genus and species are either printed in italics or underlined.

The Genus Concept
A genus (plural genera) groups species that are similar. For example, maple trees belong to the genus Acer. Thus sugar maple (Acer saccharum), silver maple (Acer saccharinum), and red maple (Acer rubrum) belong to the same genus Acer. Their leaves are similar and other features are similar but not identical. Every genus has characteristics that make it stand out clearly from other living things.

The Species Concept
Linnaeus grouped as a species those organisms that he felt were very similar in structural features. In simple terms, a single species is a distinct kind of organism, with a characteristic shape, size, behaviour, and habitat that remains constant from year to year.
A species (plural also species) is defined as a group of individuals that are alike in many ways and interbreed under natural conditions to produce fertile offspring (children).
Potato (Solanum tuberosum) and the eggplant (Solanum melongena) belong to the same genus because they are similar in many ways. However, they belong to two different species because they are not identical and they have reproductive barrier, that is, they cannot mate (cross or breed) with one another to produce fertile offspring. The members within a species can mate or cross. Thus all varieties of potatoes are in the species because they can interbreed to produce fertile offspring.