First of all, not all these titles are “royal.” You are only royalty if you are a member of the royal family, and that has a fairly specific definition. In fact, every few years, the queen tends to take away a few people’s royal status.
However, except for “king” and “queen,” you would be safe in calling these titles titles of “nobility.” There are some “royal dukes,” but even they would correctly be classified as nobility.
Part of the confusion is that the word “prince” has two meanings. One is the original meaning–used throughout Europe–of a title of nobility for someone whose domain is a “principality.” The second meaning of “prince”–specific to the British royals–is a child or other relative of the monarch (king or queen).
Prince Charles is a prince under both these meanings. He was granted the title of “prince” merely for being the queen’s son and he was also given the title “Prince of Wales.” Other relatives of the queen–her husband, Prince Phillip, and her sons Andrew and Edward–are princes only in the second sense.
Another thing you should know is that one person can have as many titles as he or she might inherit or be granted by the monarch. So, Prince Phillip is also the Duke of Edinburgh; Prince Andrew is also the Duke of York; Prince Edward is also the Earl of Wessex. These titles were also granted at the pleasure of the queen.
Note that none of these people were born with the rights to these titles. Prince Charles was made the Prince of Wales when he was a young man. Andrew and Edward were not given their Dukedom and Earldom until they were married. It’s all at the whim of the monarch.
As to why Edward got only an earldom while his brother got a dukedom – the queen had her reasons, I suppose. It’s up to her. She didn’t have to give her husband and sons any titles. If she had not, they would all have just remained royal princes. (In fact, Phillip didn’t even have the right to the title “prince” until his wife decided he should be one, beucase he was forced to relinquish all his Greek and Russian titles before he married her.)
As I said before, a single person can hold as many titles as he or she inherits or is otherwise granted. If you’re an earl and you get a dukedom, that doesn’t mean you give up your earldom; you keep both. You will probably be addressed usually according to the higher one, though.
For example, Charles is properly titled His Royal Highness Prince Charles Philip Arthur George, Prince of Wales and Earl of Chester, Duke of Cornwall and Rothesay, Earl of Carrick and Baron Renfrew, Lord of the Isles and Great Steward of Scotland.
All that being said, here are the British titles of nobility from highest to lowest
Duke/Duchess
Marquess/Marchioness (“marquess” is the English for “marquis”)
Earl/Countess (“earl” is the English equivalent of “count”)
Viscount/Viscountess
Baron/Baroness
The most prestigious forms of these titles are, obviously, royal titles, but only relatives of the monarch, such as the Duke of Kent, hold such titles. The next level of titles are those of hereditary peers, whose titles are heritable by their heirs. However, it has been a long time since any new hereditary peerages have been created.
Retired politicians (such as Margaret Thatcher), celebrities (such as Andrew Lloyd Webber) and successful businessmen are often awarded “life peerages,” usually a barony, which disappears upon their deaths.
There are additionally two non-peer titles that are granted –
Baronet
Knight
The holders of these titles are addressed as “Sir” or Dame). These are also not heritable.