In biology, a genus is a taxonomic category ranking used in biological classification that is below a family and above a species level (note: in biological classification, taxonomic categories are, from "largest" or most inclusive to "smallest" or most specific - kingdom, phylum, class, order, family [and sometimes subfamily], genus, species) and includes group(s) of species that are structurally similar or phylogenetically related.
In binomial nomenclature, the genus is used as the first word of a scientific name. The genus name is always capitalized and italicized. Example: Cervus canadensis is the scientific name for elk, with Cervus referring to the genus of elk, and canadensis referring to the species name.
Definition from URL: http://www.biology-online.org/dictionary/Genus