The norms of civilized society require certain things with regard to addressing others in discourse. For example, it is considered improper to address an adult by his or her first name unless and until invited to do so. In Yiddish, granting this permission is called dutzening. If you have not been dutzened to use a person’s first name, you use the civil convention of calling the person by title, for example: Miss, Ms., Mrs., Madame, Mr., Master, Professor, Dr., etc. There are some titles which require address by circumlocution. These traditionally include royalty (His/Her/Your Majesty), non-royal rulers of countries (Your Excellence), judges (Your Honor),* and Rabbis (Rabbi; Rabbeinu, i.e. “Our Great One;” or l’kavod haRav, i.e. “for the honor of the Rav”).
The Chakhmim (Sages) give explanation on why a rabbi is to be so addressed.
Rabbi Akiva ben Yosef taught: “Et Hashem Elokecha Tira (you shall respect HaShem your G-d – D’varim 6:13); l’Rabot Talmidei Chachamim (respect Torah scholars as well)” (b. Pesachim 22b).
Rashi taught: “You shall respect your rabbi as you respect Heaven” (Commentary on b. Pesachim 22b).
The Rambam taught: “One should not greet his Rabbi, or return greetings to him, in the same way that people greet friends and return greetings to each other, but one should bow slightly in front of him and say in reverence, ‘Shalom to you… Rabbeinu.’” (Hilchot Talmud Torah 5, Halakha 5).
In a letter, direct address is acceptable, in the form: “Dear Rabbi (surname),” though “l’kavod haRav” is preferred. Use of the first name, however, is perceived as contempt… unless that particular rabbi has dutzened you. In any formal setting, e.g. worship, though — l’kavod haRav should be used regardless of any dutzening that might define address in private conversation.
It used to be that the title of Rabbi was reserved for use in Eretz-Israel, while the diaspora Torah-teachers were given the title Rav; but today those two terms are interchangeable in most contexts. Some yeshivot will call those who have received shemikha by rabbi and those who are in the process leading up to shemikha by the title rav, treating it as lesser. In Yiddish, especially among the Ashkenaz, the title of rebbe is often preferred (as used in “Fiddler on the Roof”), and in Sephardic communities, both rabbi and chakham are used, with the latter being a bit higher in rank (on par with rabban in ancient times).
If in doubt: address everyone by title unless or until directed otherwise.