When used as titles they are different. Mr is the standard title for a man. Sir is the title of a lord, someone who has been given a knighthood. However, Sir is often used to address someone who's name you don't know - in a letter/email, for example. "Dear Sir". This is the same as using "Dear Madame" for a lady. If you know someone's name use mister (Mr). Mr Jones / mister Jones. UNLESS they have a different title, "Dr" for instance. If you don't know their name, use sir. "Dear Sir". ""Excuse me, Sir". Don't use "mister" on its own or you'll risk sounding like a cockney orphan: 'scuse me mister" :)

Hmmm! Both can be used as titles when you’re addressing someone, but “mister” is more often used together with someone’s name (e.g. Mr. Jackson). If you don’t know the other party’s name, you can use “Sir” (in emails as well). Hope that helps! :)