What is the difference between a prince a duke and an earl?

They are all Lords;

There are five levels of the peerage in the UK; 1) Dukes, 2) Marquises, 3) Earls, 4) Viscounts, and 5) Barons. ALL of them are "Lords" but not all of them are called lord in day-to-day conversation. A Duke is usually called His Grace and a Marquis (pronounced Mar-Key [it's a French Word]) [CORRECTION: it's pronounced MAR-KWISS] is called The (capital T) Marquis of X. An Earl is called The Earl of X and his wife is The Countess of X. A Viscount is called Viscount X and a Baron is called Baron X. There are other levels of nobility in the UK and in Scotland there are barons who are not called Baron X but the (lower case T) Baron "of" X to separate them from Baron X. [CORRECTION: In Scotland, Barons are not noble and a Baronage can be bought and sold. The fifth level in Scotland is actually a Lord of Parliament].

All of the above are technically entitled to be called Lord X, but a duke would have his nose put out if you did so, as would a Prince. A Scottish Baron may have in olden times been called Lord but today is not allowed to be called that due to changes in the law and custom over the years. That is why today I'm known as Fr33kman, Baron of X, but not Lord X. (ie: because I'm a Scottish baron, but not a UK Lord)

Hope this helps! :-)

You can also check http://en.wikipedia.org/wiki/Lord

[Corrections by Tom P-C]