Taxonomy is a branch of science that has to do with the naming of organisms. It is done by using a universal name that can be understood by scientists all over the world. Many times Latin is used in the naming of organisms.
Carolus Linnaeus was a botanist who lived in the 1700's and devised a system of naming organisms known as binomial nomenclature. It is a two-name system using the categories of genus and species to precisely name an organism. The genus and species name is called an organism's scientific name.
The genus is capitalized and the species is in lower case letters and the scientific name is italicized. The term genus implies that the organisms are closely related and the species name represents a group within this genus capable of only breeding with other members of the species.
An example is Felis tigris--(tiger) and Felis leo--(lion). Both are in the cat genus--Felis and have physical and behavioral characteristics associated with cats. However, these two organisms live in different habitats, have different genes and adaptations. By further dividing the genus into different species it is a more precise way of classifying those two organisms based on their differences.