~씨 : shade of meaning like Mr. ~, Ms. ~. It's a way to indicate someone formaly. ex. When you want to call '김민수' formally, usually you might say '민수씨' ~님 : much polite than ~씨. Normally this word is used for VIP, President or something like that. But in service industry, the company forces their employees to call their customer as '~님'. Bascially it has meaning of respect. ex. 교수님(Professor) is used when university student indicate their Professor with respect. But just as a kind of job, or without respect people call Professor as 교수 ~야 : it isn't a way to call someone formal, inversly, it is used between young friends, or when mom or dead call their children ex. When 김영희(Mother) call 김철수(Her Son) -> 철수야~