A: Rabbi was the title given to those ordained with the original ordination, that died out in the fourth century. Scholars who were officially ordained, were called simply “Rav”. In modern times, rabbis are callled “Rav” in Hebrew, whereas the term “Rabbi” is more common when speaking English. Reb is a title used in Ashkenazi communities for a respected person, not necessarily a rabbi. Rabban (Aramaic for “Our Master”), was the title of the heads of the Sanhedrin. Rabboni is not so much a title, but an adjective placed before someones name, indication that he is a scholar. Rebbe is the title of Hasidic leaders, who primarily give spiritual guidance and inspiration. A Rebbe may or may not be an ordained rabbi.
(Answer from Rabbi Ya’akov HaLevi)