The Holy Temple was destroyed millenia ago, but Conservative and Orthodox Jews still acknowledge the three-fold division of ancient Israel into Kohanim, Leviim and Yisraelim. Reform Jews do not believe any congregant should have a different status than another, and therefore do not acknowledge these divisions.
These groupings began with the division of the Jewish nation among the twelve children of Jacob, one of whom was Levi. The tribe of Levi was designated for a special role of Divine service. When the Torah addresses the Levites at the beginning of Numbers, they are set apart from the rest of the Israelites. God tells Moses not to include them in the census of the tribes, and the Torah lists no chief as their leader. Their function is not to defend the camp or participate in the conquest of the land, but to guard the Tabernacle from human defilement, "Any outsider who encroaches shall be put to death" (Numbers 1:51).
The Levites are counted in their own census, and their numbers are far smaller than those of any one of the 12 tribes. The Levites were the smallest of the tribes. Pharaoh permitted this clan to be Israel's spiritual leaders as opposed to laboring as slaves in Egypt. According to Midrash, to frustrate Pharaoh's will, God made each tribe increase proportionally to its suffering, but the Levites did not suffer as much as the other tribes, and therefore did not multiply to such a great extent.
Some Rabbis contend that God excluded the Levites from the general census because God foresaw that unlike their brethren, they would be superior to their peers. They would not despair at the terrifying reports of the spies returning from Canaan (Deuteronomy); therefore, they would not die in the wilderness, but live to enter the Promised Land. Had they been listed in the general census, the angel of death would have killed them with the other Israelites.
Maimonides portrays the Levites as a universal ideal for people of pure faith, ready to abandon the world and cultivate a life of inner tranquility and supreme wisdom. According to Maimonides, every individual whose spirit moves him and whose intelligence gives him the understanding to withdraw from the world to serve God and to know God, and who walks upright in the manner in which God made him, is totally consecrated, and God will grant him/her a place in the World to Come.
From the tribe of Levi, Aaron and his descendents were singled out to be the priests and serve in the Temple (see Exodus 28). Originally, God intended that the first-born of every Jewish family would be a Kohen, the priest who would serve as that family's representative to the Holy Temple (Exodus 13:2, Exodus 24:5). The decision to make the descendants of Aaron serve as priests, however, occurred after the Golden Calf incident at Mount Sinai (Exodus 32:26). At Sinai, only the Levites remained loyal to G-d by not worshipping the Golden Calf, and therefore the Levis were designated as the priestly tribe.
Today, Leviim are believed to be the direct patrilineal descendents of Levi, while Kohanim are Leviim who descend directly, through their fathers, from Aaron. Other Jews are assumed to come from one of the other tribes and are called, simply, Yisraelim. A convert to Judaism takes the status as a Yisrael. The only valid method of being a Levite (or Kohen) is to have an unbroken tradition, passed from generation to generation, stretching back to the time of Moses. In many Jewish communities, meticulous records were kept throughout the generations to ensure that ancestral lines remained clear. If one has no clear evidence, such as a family tradition, of descending from Levi or Aaron one should assume he/she is a Yisrael. Traditionally, women adopt the status of their husband, thus if a bat Kohen (daughter of a father who is a Kohen) marries a Yisrael, she and her children are Yisraelim.
Today in many synagogues, the first two aliyot (people called up to the Torah) are given to a Kohen and a Levi. Yisraelim, the majority of Jews, are called to the Torah only after the second aliyah.
In old cemeteries, a pair of hands symbolizing the priestly benediction often marks the tombstone of a Kohen, while the grave of a Levi, who poured water over the hands of the priests before the recitation of the blessing, is signified by a tilted pitcher.
According to the Talmud, the number "3" is a basic pattern of the revelation experience. In the third month of the year, Sivan, God revealed at Sinai a text divided into three sections, Torah, Prophets and Writings to a nation divided into three groups, Priests (Kohens), Levites and Israelites through Moses, the third child after Miriam and Aaron, on the third day of Israel's preparation for revelation.