Jacob, whom God later renamed Israel, had 12 children. These 12 children and their descendants became known as the 12 tribes of Israel.
Levi was Jacob's third son, born of Leah. Levi means joined. Leah named him Levi hoping that now her husband would be joined closer to her. Interestingly enough, joining people closer to God has become the life work of the tribe of Levi.
When Jacob was preparing to die he called his sons to him. Of Levi he could not say anything good. Jacob stated "Levi, an instrument of cruelty. May mine honor not be united with them for in anger they slew a man, and in self-will tore down a wall. I will scatter them in Israel." Levi received these statements because when his sister Dinah was taken and raped by the prince of Shechem, Simeon and Levi deceived the people of Shechem into a false truce and then slaughtered all the men of the city.
This trait of vengeance and anger was also manifested in Moses, who was of the tribe of Levi. When Moses was in Eygpt he saw an Egyptian man hitting an Israelite man. Moses, in his own anger, killed the Egyptian. God had to allow Moses to be a shepherd in the wilderness for 40 years so that he could learn that our ways are not God's ways. God says that vengeance belongs to Him, not to us.
The tribe of Levi also learned to stay close to God and follow His directions. When Moses came down off the mountain with the Ten Commandments and found all the people worshipping a golden calf, he called out, "Who is on the Lord's side? Let him come unto me." The Bible records that the sons of Levi came to his side. They showed their repentance and stood on God's side during a crisis, when others were departing from the truth.
Moses, when he was about to die, said the most wonderful blessing upon the tribe of Levi. God then turned Jacob's curse, which stated that they would be scattered in Israel, into a blessing. The Levites became the spiritual leaders and teachers, living scattered among all the other tribes.
Picture originally found here