Dear is used when addressing a letter, but can also refer to somebody else (used much less often)
Darling is used to reference your wife, but can also refer to somebody else. This word conveys a stronger sense of 'love'.
Dear "Company Name". I am writing to inform you....
Waitress says "What would you like dear?"
Have a good day darling
Waitress says "What would you like darling" (However, this case is used much less often"