Ancient: Egypt; Greece; Rome; etc. - the ancient world.
Dark Ages: from the fall of Rome (410 AD) to 1065.
Medieval: from 1066, up to and including 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: the wild, wild West! Contemporary or historical.
Colonial: early settlements, exploration, and Southern romances (including Americana).
Victorian: from 1830 to 1901 (including William IV's reign).
Civil War: America - from 1861 to 1865 (the civil war era).
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)
Historical: from the dawn of time to 1969 - any location.
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. Contemporary or historical.
Anthology: must contain at least one genre from above.