In the eighteenth century, when Linnaeus invented the naming system that is still used in biology and botany today, he expected that every plant would have a unique and universally recognized two-word name: the first (genus) capitalized and the second (species) not. Spiraea japonica refers to the same plant the world over.
But sometimes differences arise, as a result of natural mutation or uncontrolled cross-pollination. These differences may be important but not unique enough to justify a new species name. Such plants are called varieties, and their differences are likely (but not guaranteed) to reproduce well from seed. Varietal names, in italics, are not capitalized and simply follow the species name, sometimes set off by “var.” Spiraea japonica albiflora is smaller and paler than its parent.
Cultivars (a contraction of “cultivated” and “variety”) are plants that have been bred or selected for desirable characteristics. They are propagated vegetatively, from cuttings rather than seeds, because their seedlings generally do not reproduce the parent. Cultivar names are capitalized, enclosed in single quotes, and not italicized. Spiraea japonica ‘Goldflame’, with bright yellow leaves and bright pink flowers is considerably more colorful than Spiraea japonica.
All this helps to assure that everyone — no matter what language they speak — can speak the same language when they want to identify a plant.