Peerage

A peer is a member of the nobility.  It is sometimes used instead of 'Lord'. In formal or old British documents, the House of Lords is called the House of Peers.

Ranks 

In the United Kingdom there are five ranks of the peerage:

 Baron is the lowest. In Scotland this is called a Lord, short for Lord in Parliament.
 Viscount
 Earl - this is an old Saxon word. In Continental Europe this rank is called 'count', the lord in charge of a county. An earl's wife is called a countess
 Marquess - A special rank higher than an earl because a marquess's land was in the Marches, the border areas that must be defended. A marquess's wife is called a marchioness.  There were not many marquesses in Scotland, and they usually spelled the title 'marquis' like the French
 Duke - the highest rank.

Informally Barons, Viscounts, Earls and Marquesses are called lords, and instead of their name when speaking to them, the term 'my lord' is used. A Duke is never called a lord. 'Your grace' is used for a Duke.

Since 2004 a list of peers has been kept by the Lord Chancellor and Secretary of State for Justice. This list, called the Roll of Peerage had to be kept to prove who was a peer.  The list of members of the House of Lords used to be the same until the hereditary peers were excluded. Official documents can only call someone a peer if their name is on the Peerage Roll.

 Precedence 
Sorting out which peer should precede (come in front of) in a procession or other event depends on three different things:
The rank.  All dukes come before all marquesses, then earls, then viscounts and finally barons.
Country the peerage was created in.  All Dukes first created by the King of England come before those created by the King of Scotland.  After this come Dukes of Great Britain whose titles were created between 1707 and 1801 (during the existence of the United Kingdom of Great Britain). In 1801, the kingdom of Ireland joined with Great Britain to form the United Kingdom of Great Britain and Ireland, so Dukes of Ireland come next, followed by Dukes of the United Kingdom. The other types of peer follow in the same order.
England
Scotland
Great Britain
Ireland
United Kingdom
Finally the order is decided on the year the title was created.

 Exceptions 

The only exceptions are the royal dukes, and the Duke of Abercorn.
The dukes of Gloucester, Kent, Edinburgh, York, Cambridge, and Sussex are the most junior dukes, but because they are princes they rank ahead of all other dukes.  In 1952 the Queen ordered that her husband, the Prince Philip, Duke of Edinburgh, would be second only to her. The Duke of Edinburgh is now Elizabeth II's son, Prince Edward, Duke of Edinburgh.
The Duke of Abercorn.  When the Irish Marquess of AbercornPeerage Roll 2004  Peerage Roll lists the Marquessate as a UK peerage and notes that the peer is usually called by a higher title.  This is usual with Irish peers was "promoted" in 1868 the duke was listed as a duke of the kingdom of Ireland, but was given precedence as if it was a peerage in the Peerage of the United Kingdom. Irish peers cannot attend the House of Lords.

 References 

Category:British nobility
Category:Peerages