When you visit a garden centre or look through a catalogue you can be a bit mystified by the terms, especially the Latin names. Every plant has two Latin names. Take an example, Camelia sinensis is the plant from which we obtain tea. Camelia, which begins with a capital, denotes the genus, the group of plants to which it belongs, and sinensis denotes the species. This is not given a capital. There are other species in the Camelia genus, for example, Camelia sasquana, which is used in Japan for a kind of tea. It is important to realize that plants within the same genus can produce fertile hybrids. For example there are several species in the genus Amelanchier, a kind of shrub. Amelanchier canadensis x grandiflora is a hybrid between two species, canadensis and grandiflora. This is an interspecific hybrid, a hybrid between two species in the same genus. Yet you can also have intergeneric hybrids, which occur when plants from two related genera [plural of genus] are bred together. One example is leylandii, which is x Cyprocyparis leylandii, which is a cross between a Nootka cypress and another member of the cypress genus. Note that intergeneric hybrids have the cross before the name rather than within it. They are not fertile and have to be propagated vegetatively, by cuttings.
Sometimes a single species contains much variety. For example, Brassica oleraceae, has several distinct varieties. Cabbages, kale, broccoli. calabrese,cauliflowers and sprouts all are varieties of this single species.
Various related genera are classed as families. Some families are small and have few genera in them, but some are huge. The rose family, the Rosaceae has a hundred genera and two thousand species; and you would not think that some belong to the same family. The family includes roses, but also Rubus [blackberry and raspberry] Prunus [plums] and Frageria [strawberries.] The Rose family belongs to a wider group of families which are grouped into an order, the Rosales. You will not need to bother with the term order when buying plants for your garden. There has never to the best of my knowledge been any interfamily hybridization
Yet of more importance to gardeners are divisions within species. You often find varieties. For example, there are red, white and yellow roses. These may all be part of the same species, but are different varieties within the species. They interbreed easily with other varieties in the same species. There will be a variety name, for example Amelanchier canadensis x grandiflora 'ballerina' is a specific variety of that hybrid. Note that varieties do not have Latin names. One kind of variety is a subspecies. This is usually a variety within a species that is strongly linked to an area. For example,the cricket bat willow, Salix alba caerulea thrives best in eastern England. The plant name includes sspc before the term denoting the subspecies. Note that a subspecies can have a Latin name.Hybrids between different subspecies and varieties are known as infraspecific hybrids. However, while this term is used in botany, we do not use the term infraspecific hybrids in talking of mixing between breeds of animal or races of human.
A specific kind of variety is a cultivar. This is a variety that is kept distinct by human effort, which involves ensuring that interbreeding with other cultivars does not take place. Propagation of cultivars is rarely by seed and is generally by vegetative methods, such as cuttings, layering and division.
Below the level of cultivar there is form and subform, the latter being the smallest division of a plant cultivar. Forms and subforms are so slightly different from other members of the variety to which they belong that they are easily lost when plants interbreed, so they have to be maintained artificially by vegetative methods. Forms and subforms only occur in the world of ornamentals. Vegetable growers do not need these terms.