Carolus Linnaeus was instrumental in developing a system for the classification of living things. It is known as binomial nomenclature. It is still used today.
Each species is assigned a two-name scientific name that is universally understood by all scientists. It is generally written in Latin. By the 1700's, the names that scientists used to classify organisms were generally long and very confusing. The system devised by Linnaeus made taxonomy (the naming of organisms) much simpler.
The genus name is assigned because it represents a group of closely related species. An example of this is the genus Canis. This represents organisms that are in the canine group including dogs, jackals and wolves.
The next part of the scientific name is the species name. The definition of a species is a group of organisms who can interbreed. Canis lupus is the scientific name for gray wolves.
During Linneaeus' time organisms were categorized into categories from most general to most specific in the following order: kingdom, phylum, class, order, family, genus, species. Based on evidence of the time, classification was mainly done based on similarities that could be observed. Today, evolutionary relationships, DNA evidence, etc. help scientists to classify organisms in a more precise way.