A hierarchy of important ranks
A genus is a rank in the biological classification (or taxonomy). It stands above species, and below families. A genus can include more than one species. When biologists talk about a genus, they mean one or more species of animals or plants that are closely related to each other.
As with other taxons, the plural is different from other English words because it is a Latin word. 'Genus' is the singular, and 'genera' is the plural form of the word.
When printing the scientific name of an organism, the name is always in italic. A name of species has two parts, with the genus first. For example, in "Felis silvestris", Felis is the genus. The genus name always begins with a capital letter. In "Felis silvestris catus", the third word is the subspecies, which is not often used.