The word μπάρμπας comes from the Venetian (and Latin) word for "beard" and in Greek means either "uncle" or "old man".
In colloquial speech however, it is more often used to mean "old man" and rarely to imply an actual blood relationship. For example "μπαρμπα-Αντώνης" can be used for an old man that is called Αντώνης and is a familiar face in the neighbourhood. Many people may refer to him like this even though he is not a relative. The word may also be used as an insult to imply old age/stupidity. Example "Προχώρα ρε μπάρμπα!" (Move up you old geezer!).
The word κύριος is of Greek origin and (as a noun) is translated as "Sir"/"Mister" or "Gentleman".
It is the generic term to put before a male name (or surname) if we are not familiar with the person, or to show respect. It is considered polite and the same word is sometimes used to refer to God (as "the Lord").
Use "κύριος" to refer to others and "μπαρμπα" only when you are 100% sure that wont be taken as an insult.
