1. The Posek HaDor means literally the "Decider of the Generation". It is the title given to pre-eminent authority in Jewish Law. Today in the Ashkenazi world the Posek HaDor is Rabbi Elyashiv and in the Sefardi world it is Rabbi Ovadia Yosef.
2. Maran means "Our Teacher" and it is a formal title of respect for a teacher. In many cases the Posek HaDor will also be called Maran.
3. Shlita is an acronym for "SheYichye l'Yamim Tovim, Amen" which means that "He Should Live a Long Life, Amen". It is normally found at the end of a Rabbi's name.
4. Segula means a treasure and is only rarely used in connection to a person although sometimes when very flowry language is being used a Rabbi might be called a Segula.
5. Zatzal is annacronym for "Zecher Tzaddik l'Vracha" which means that "The Righteous One Should be Remembered as a Blessing"
6. HaGaon means literally the "Great One". Its roots are very early as there was a Gaonic era just after the Talmudic era. Today it is a title that is given to a great and exceedingly knowledgable Rabbi.