Medieval: romances before 1066, and up to the 17th century.
Georgian: from 1702 to 1811 (the age of reason and freedom).
Regency: from 1811 to 1830 (includes the regency and the kingship).
Western: nothin' but the wild west, and handsome gunslingers!
Colonial: early settlements, exploration, and Southern romances (including Americana).
Victorian: from 1830 to 1901 (including William IV's reign).
Vintage: from 1901 to 1945 (Edward VII, World War I, World War II, etc.)
Nostalgia: from 1945 to 1969 (Korean War, Beatniks, Vietnam War, Hippies)
Celtic: historicals / paranormals / fantasies that take place primarily in Scotland, Wales, Ireland, Brittany, etc.
Time Travel: modern-day heroes and heroines thrown back in time to meet their true loves.
Fantasy: dragons, fairies, wizards, and witches - oh my!
Paranormal: vampires, ghosts, werewolves, shape-shifters, goth, etc. Historical or contemporary.
Anthology: a collection of short stories written by several romance writers (must contain at least one genre from above).