Previous answers are correct, but ... 1) 'sera' means 'evening' and 'serata' means 'evening and what happens in it', so, for example) you mostly use 'buona serata' when you are saying good-bye to somebody that goes to a party, to a dinner with friends, to see a movie, etc. In other cases, when you think that the other person's evening is quite plain (but could nonetheless be very interesting, like reading a fine book), I would use a plain 'buona sera', or, obviously, just 'ciao' as with friends. 2) You don't say 'buona sera' to friends, BUT you can say 'buona serata' to them, this is related to previous note (1).

Buona sera si usa quando si incontra qualcuno e buona serata quando si va via.

Buonasera e buona serata mean Good evening but Buonasera is used when you meet somebody , buona serata is used, instead, when you leave somebody/ a place