> Lord 

    The noun Lord was a title  used to address some upper-class people of British Royal Families or given by the government to some high Scholars. Lord is also used as an inherited term to denote Peerage.

● Five Ranks of Peer exist in England (In Descending order)

   1) Duke

   2) Marquess

   3) Earl

   4) Viscount

   5) Baron

   The title Lord is most often used by Barons. They are rarely addressed by their formal and legal title Baron. The Lord title generally refers to Marquess, Earl, Viscount, etc. Dukes are commonly addressed as The Duke of X (Name of Region) and instead of My Lord,  " Your Grace " is used to respect them. 

   Elder sons of Duke, Marquess also use the Title Lord or elder son can use his Father's Subsidiary Title (if any).

   People who are Members of the House of Lord ( the upper house of the Parliament of the United Kingdom ) and some judges who sat in the House of Lords also get addressed by the title of Lord (until the creation of Supreme Court of UK, 2009 ).

   Some Governor-General of British India and Viceroys referred to as Lord.

> Sir 

    Sir Title is generally used to address a man who has the rank of Knight or baronet. In an easy sense man who fights to protect Britain and its realm and is part of the British Army would be addressed as Sir. Sir is a formal English Honorific address for man, derived from a sire in the high middle Ages.


Look at the above photo, you will find some quality differences between Lord and sir.