Language/French/Vocabulary/How-to-Say-Good-Bye
Informal
Salut !
Formal (when you will perhaps meet again)
Au revoir !
Formal (when you will never see that person again)
Adieu !
Informal (when you will perhaps meet again)
A plus ! / A plus tard !
"à plus" or "à plus tard" is used when it is safe, or very likely, that people will meet again in the same day (or evening), but the time and conditions are not necessarily agreed.
A la prochaine !
"A la prochaine !" is generally used with friends, or when we are sure that there will be a "next" without having to worry (appointments or regular meetings). When the conditions of a new meeting are more uncertain, one can opt for the variant to a next.
Formal/Informal (adding a temporal precision, when you will meet that person again)
A demain !
A tout à l'heure !
"A tout à l'heure !" refers to a particular moment (in the hours that follow) that the interlocutors know how to identify. It is not necessarily very precise. For example, it may be "the moment I come back after doing what I have to do". The variant immediately indicates a return or (on the phone) an imminent arrival.
A bientôt !
"A bientôt !" insists that we hope to see our interlocutors again soon (it may be a real feeling or a polite mark of attention). In particular it is appropriate when it is not at all obvious that a new meeting takes place, in this case it can be followed by "I hope".