Levi, the third son of Jacob, was a passionate man, whose zeal, when transmitted to his descendants, was focused toward God.
According to the Midrash, unlike the other Israelites, the Levites were exempt from slavery by the Egyptians. They remained at home, studying laws passed down from Jacob and were thus able to inspire and teach their fellow Israelites.
At the foot of Mount Sinai, however, their zealousness truly distinguished them from all other Israelites. Upon seeing the Golden Calf, Moses called out “Whoever is on God's side, [come] to me” (Exodus 32:26). Only Levites responded and, following Moses’ instructions, punished the idol worshipers. For their extraordinary loyalty, the Levites were specially consecrated to God.
Before his death, Moses blessed the entire nation and each of the tribes independently. Moses praised the Levites for their devotion to God, which was later transformed into the drive to educate the Children of Israel about how to serve God.
The Levites had already been designated to serve as ministers in the Tabernacle (and later in the Holy Temple). While the priests (Kohanim) performed the actual sacrifices and rituals, the Levites guarded and maintained the Tabernacle/Temple. They also served as the choir and orchestra whose songs and music accompanied the sacrifices throughout the day.
Because of their ongoing involvement with the Tabernacle/Temple and Jewish education, the Levites were not given a portion of the land. In return for their services, they received a special tithe of 10% donated from all agricultural produce of Israel.
Today, when there is no Temple in which they might serve, the Levites retain a minor role in the synagogue, receiving the second aliyah (the honor of reciting the blessings) at the Torah reading and washing the hands of the Kohanim before they recite the Priestly blessing.