By the end of the 17th century, plants were grouped into genera and the species within the genera were described using Latin phrases. Plants we call oaks, for example, were all referred to as Quercus (the genus), and the different species of oak (e.g. white oak, black oak, etc.) were designated by descriptive, lengthy Latin phrases. 

By the mid-eighteenth century, Carolus Linnaeus, a Swedish physician and naturalist, developed a system of naming in which the Latin phrase that was used to designate species was replaced with a single word. This system of binomial nomenclature (system of two names) is still in use today. 

Plants are referred to as belonging to a particular genus and as a species within that genus, e.g. Quercus alba (white oak). The species can be further subdivided into subspecies and varieties in some cases, and this refinement results in an extra name that follows the species name.