Comes from the Latin, "imperator," which was originally a military title. Soldiers would salute the leader of a victorious army as "imperator." Augustus Caesar assumed the title and all subsequent Roman and Byzantine leaders. In Europe, Charlemagne became emperor of the Holy Roman Empire in 800. Various countries, including Russia, China, Japan, Persia, the Ottoman Empire, Ethiopia, and India, have used the title "emperor."
In 1804 Napoleon was named emperor in 1804. Except for the phrase "Emperor of India," which was added to the British Monarch's title in 1877 and used until India became independent, Britain has not used the term.
The monarch outranks everyone else. In England, which generally gives precedent to males, the Queen nonetheless outranks her husband since she inherited the title. Queen Elizabeth's husband is Prince Philip, Duke of Edinburgh. Men who are commoners, such as Antony Armstrong-Jones, who married Princess Margaret, typically receive a title as a courtesy. Armstrong-Jones was made the Earl of Snowdon.
In England, a prince or princess are often children of the monarch, and therefore royalty, as opposed to nobility, but those holding the title can vary in rank. In France, a duke, or "duc" outranks a prince. In Russia and Austria, the title archduke is an indication of royal blood, and is used instead of prince.
Originally all English dukes were of royal blood. When sons of kings came of age they were typically given the title duke. Now a duke is the highest of the five degrees of English nobility.
Other noble titles, indicating one is a member of the hereditary peerage, are: marquess, earl (in France and elsewhere on the continent, "comte" or count), viscount, and baron.
A baronet is not included among the peerage, but the title can be inherited. Below a baronet, is a knight, which is a title of honor rather aristocracy.
While the hereditary rights of the British aristocracy have diminished over time, peers still retain the right to vote in the House Lords, the upper house of Parliament.
While titles can be inherited, the Life Peerages Act of 1958 permitted the creation of non-hereditary lifetime titles, whose holders, but not descendants, are entitled to vote in the House of Lords.