Following the binomial system introduced by Carl Linnaeus in the 1700’s the they appear after the specific epithet (second term in the scientific name) almost final piece of the puzzle and a clear identifier for any plant is it’s variety or cultivar. The main difference between the two is that a VARIETY is a usually wild version and a CULTIVAR is almost always a specifically cultivated version.
VARIETY – Digitalis purpurea var. alba
CULTIVAR – Digitlalis purpurea ‘Sutton’s Apricot’
A variety typically breeds true from seed while cultivars may or may not.
Variety is always written in lower case often preceded by the abbreviation var. while a Cultivar name is usually bound in single quotation marks the first letters capitalised.
Oh and a plant can have both Variety AND Cultivar