The priestly laws are described in the Book of Leviticus.
The Book of Leviticus is so named because it contains a great deal of information regarding the priestly house of Levi, specifically the children of Aaron, known as the Kohanim (singular: Kohen, also spelled Cohen). Other descendants of Levi, known as Levites (Hebrew: Levi'im, singular: Levi) are enjoined to assist the Kohanim in various tasks, participating in rituals both in the era of the Temple in Jerusalem and in the modern day.
Temple-Era Duties
The Kohanim were required to perform most of the duties related to the sacrifices offered by the Jews in the ancient Temple of Jerusalem, including animal sacrifices, meal sacrifices and incense burning. The Levites carried out many of the secondary tasks, such as opening and closing the large doors and providing musical accompaniment on festive occasions. With the destruction of the Second Temple in the year 70, Kohanim and Levites no longer perform these activities, although their laws are still studied by pious Jews today in anticipation of the rebuilding of the Temple.
Blessing the Nation
Kohanim were also required to bestow the Priestly Blessing on the people (Numbers 6:22-27). Levites would ritually wash the hands of the Kohanim, who would then drape a prayer shawl over their heads and recite the verses before the assembled people. This ritual is still performed daily in Jerusalem and on major holidays outside of Jerusalem.
Assessing Contamination
The Kohen also performed the quasi-medical function of assessing the level of contamination related to the plague known as tsara'at, sometimes rendered in English as "leprosy," although this is unlikely because the plague, unlike the skin disease, would affect houses and clothing as well (Leviticus, chapter 14). Kohanim would examine infections, declare quarantines, and release individuals from quarantine as necessary.
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