The Queen has quite a few titles "held" by the Crown not being used. She gives these out, usually when they marry.
King/Queen
Ruler of the Kingdom and head of the Nobles.
Prince/Princess
Usually the son or daughter of the King or Queen. Can be (usually) the first cousins of the king or queen. Most Dukes are considered Princes or can be.
Dukes/Duchess
Rules the Duchy or Ducal Kingdom, usually for a King or Queen. However in Medieval times they were often the equivalent of a King or Queen. Since they only had the one area to rule, they weren't considered Kings or Queens. Usually this title is reserved for members of the Royal family (or the Royal family from a generation before).
Marquess/Marchioness
Just below a Duke in importance and above a Count (Europe)/Earl (England). Usually, he was considered more important because he could be counted on to protect the kingdom from outside invaders, thus his lands were usually on the outside part of the Kingdom. Their lands are called Marches.
Count/Earl/Countess
They rule counties. This is where the term came from to describe the areas that make up most US States (except Louisiana and Alaska where they are called a Parish). Usually a few counties were involved in a Ducal area. Basically he ruled an area in place of the king (like a viceroy). He was basically a Govenor.
Viscount/Viscountess
Basically, the children of many Counts/Earls, etc., are often viscounts. Princess Diana's brother was a viscount until the death of his father, Earl Spencer. It can be an honorary title, earned.
Baron/Baroness
They are the Kings "men." That is they are created by the King and work to his good. They are also called "lord or lady." Their lands are Baronies.
Lord/Lady
They are the children of Nobles, such as those above or are hereditary titled, meaning its passed from one generation to another.
Knight/Dame
Called 'sir.' They are a member of the Warrior class in the Middle ages. Today they are chosen by different criteria such as earned by a lifetime of achievement. The can only be knighted by Queen Elizabeth if they are British, otherwise they are honored another way. Then they recieve an OBE, MBE etc., but not the title Sir/Dame.