In today's religious systems, at first glance there would seem to be no difference. However, there are many and they are important. In Judaism, a priest is born to the Levitical line of Aaron the brother of Moses. Aaron's line is the Levitical priesthood. They are the only ones authorized to make the Temple sacrifices and intervene before God for the people. The book of Leviticus outlines the duties, requirements, and obligations for the entire family of a Levite who serves. The priesthood was established when the sons of Levi did not partake in the incident of the Golden Calf. It is a bloodline obligation, and the sons of Levi are caretakers of all the things that belong to God..that is the Temple itself, the instruments of the Temple, the services in the Temple and the purity of the people. Since there is currently no Temple in Jerusalem, there is no active priesthood. Those whose name is Levi, Cohen, Kahane, and other variants are possibly in the Levitical family as these are traditional names for Levites. A Rabbi is a teacher. That is what the word means, teacher. A rabbi can be anyone who teaches another in the principles of Judaism. In order to become a Rabbi, there are formal courses at "university" called "yeshiva" for the title of Rabbi. In short, a Rabbi is a religious teacher. As in most religions, there are greater and lesser rabbis and one must find a Rabbi who will teach the most effectively. Today's system is not like the greater yeshivas of ancient days. The yeshiva system of today does not generally recognise certain central figures such as Rambam or Rashi as great teachers who lead the yeshiva. The Rabbi is the religious leader of the synagogue. The synagogue is the center of the religious life of the Jew since there is no Temple in Jerusalem. The rabbi attends to "sermons" and questions of Jewish law known as halacha. He does not attend to the day to day operations of the synagogue; that is the responsibility of the president of the synagogue.