A genus is a scientific way of showing that species are very closed related to each other. In fact the first word of the species' scientific name is its genus. So for lions (Panthera leo), Panthera is the genus and tells us that they are closely related to tigers (Panthera tigris), because they share the name.
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories. The rank of species, and specification of the genus to which the species belongs is basic, which means that it may not be necessary to specify ranks other than these.