Nomenclature is the method biologists use to precisely define the relationship of an organism. Common names are not descriptive enough. When I say "grass", more than one thing comes to mind, so you need a way to narrow it down.
The genus is always capitalized and the species is not: Cannabis indica
When the genus is mentioned frequently, it is often abreviated: C. indica
When there is a sub-species it is distinguished like this: C. indica ssp. kafiristanica
A variety is indicated by: C. indica var. afghani
A land-race, or "form", isn't different enough to be a variety and it is written like this: C. indica f. maple-leaf
Any plant that is in cultivation (a cultivar, clone or hybrid) is capitalized and seperated by quotation marks: C. indica "Super Skunk"
Inbred lines are indicated by an alpha-numeric figure: NL#5
There is disagreement among biologists as to whether Cannabis should be divided into species or not, but for practical purposes we usually divide it into C. indica, C. sativa, and C. ruderalis. C. indica has wide leaves and either a cadelabra shape or a dominant main cola. C. sativa is a tall plant with many branches and is either bamboo shaped or christmas tree shaped. C. ruderalis is a small plant with little THC that flowers independant of the photoperiod.