A Group is united by some common trait; for example there may be a Group of yellow-flowering cultivars, a Group of cultivars with variegated leaves, a Group of cultivars resistant to a particular disease, etc. A cultivar may belong to more than one Group (for example, it may be yellow-flowering, with variegated leaves and resistant to the disease at one and the same time).
The ICNCP defines a standard form of nomenclature for cultivar groups. A group epithet, each word capitalised, is followed by the word Group. It follows the species binomial name, appearing un-italicised. Eg. Brassica oleracea Capitata Group