Which is the proper path for man to choose for himself?
Whatever brings beauty to he that does it
and is beautiful to mankind.
Be as careful with a minor mitzvah as with a major one,
for you do not know the gift of reward for mitzvot.
Consider the loss caused by a mitzvah against its reward,
and the gain of a sin against the loss it incurs.
Reflect upon three things and you will not come to sin. Know what is above you:
an Eye that sees,
and Ear that hears;
and that all your deeds are recorded in a book.
Pirkei Avot 2:1
Rabbi is R. Yehuda HaNassi ("the prince"), author of the Mishnah.
Rabbi is credited with preserving Judaism for all time by writing the Mishnah and setting down a universally accepted standard of Jewish law. Because of the Roman persecutions during the generations that preceded Rabbi, Torah study had been in decline and many disagreements in Torah law arose that had not been resolved. But through his friendship with Anthony Caeser, Rabbi gained some respite for the Jewish nation and was able to gather together all the sages of Israel in order to resolve the disputes and set down a unified code, which became known as the Mishnah.
He is known simply as Rabbi, Teacher (see Shabbat 32b), since his function as leader and teacher defined him--it became his name (the Rebbe).
Holtzberg Memorial Edition