Naming Cactus and Succulents ... 5/5

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A species is all those plants in nature that are able to reproduce with each other (and not form hybrids) because they are botanically similar. An example would be all the saguaros in the world; they are all one species (Carnegia gigantea) and are native to Arizona but found in cactus collections around the world. Other similar cacti (say organ pipes - Stenocereus thurberi) are not that species though they look somewhat similar and also native to Arizona down into Mexico, but are different biologically and can't reproduce with the other (or form hybrids if they do) species.

A breakdown from Kingdom to species level for Stenocereus thurberi
Kingdom: Vegetable i.e. not an animal
Division: Spermatophyta (seed bearers) (ends in -phyta)
Superfamily: Angiospermae (flowering plants)
Order: Caryophyllales -ales
Family: Cactaceae (Cactus Family) -acaea
Subfamily: Cactoideae -oideae
Tribe: Pachycereeae -eae
Genus: Stenocereus
Species: thurberi

Common name: Organ Pipe Cactus (an unofficial name that may not be specific to that plant)

Further divisions:
Varieties: littoralis (plants are distinguishable by being smaller in size than the "normal" organ pipe)

Additional categories can be added, where needed, into the system above. For example (mammillaria) a large genus can be divided up into subgenera, a sub genus into sections and a section into series.
Some words such as `kind', `sort' or `group' have no special rank and usually indicate "horticultural" varieties.

Further reading in the wikipedia