The main difference between Ser and Sir is that the is gender neutral, as opposed to "Sir and is a way of setting Westeros apart from Medieval Europe. Also, ser gives the book more of a sense of environment, and setting. It's just a form of a figurative language to help the tone and setting of the story and Sir is a it is used as a polite or respectful way of addressing a man, especially one in a position of authority.
Sir
Sir is an honorific address used in a number of situations in many anglophone cultures. The term can be used as a formal prefix, especially in the Commonwealth, for males who have been given certain honours or titles (such as knights and baronets), where usage is strictly governed by law and custom. The term is also commonly used as a respectful way to address a man, usually of superior social status or holding a commissioned military rank. Equivalent terms of address to females are 'ma'am' or 'madam' in most cases, or in the case of a young woman, girl, or unmarried woman who prefers to be addressed as such, 'miss'. The equivalent term for a knighted woman or baronetess is Dame, or 'Lady' for the wife of a knight or baronet.
Ser (noun)
An address or courtesy title to any person, especially if their gender and/or form of address are unknown.
"Would ser care to dine this evening?"
Sir (noun)
A man of a higher rank or position.
Sir (noun)
A respectful term of address to a man of higher rank or position, particularly:
Sir (noun)
to a knight or other low member of the peerage.
"Just be careful. He gets whingy now if you don't address him as Sir John."
Sir (noun)
to a superior military officer.
"Sir, yes sir."
Sir (noun)
A respectful term of address to any male, especially if his name or proper title is unknown.
"Excuse me, sir, do you know the way to the art museum?"
Sir (noun)
Used as an intensifier after yes or no.
"Sir, yes sir."
Sir (verb)
To address (someone) using "sir".
"Sir, yes, sir!
Don't you sir me, private! I work for a living!"
Sir (noun)
A man of social authority and dignity; a lord; a master; a gentleman; - in this sense usually spelled sire.
Sir (noun)
A title prefixed to the Christian name of a knight or a baronet.
Sir (noun)
An English rendering of the LAtin Dominus, the academical title of a bachelor of arts; - formerly colloquially, and sometimes contemptuously, applied to the clergy.
Sir (noun)
A respectful title, used in addressing a man, without being prefixed to his name; - used especially in speaking to elders or superiors; sometimes, also, used in the way of emphatic formality.
Sir (noun)
term of address for a man
Sir (noun)
a title used before the name of knight or baronet
