The priests officiated at the altar of sacrifice. There were actually a multitude of priests from a long line of priestly families whose genealogies were recorded in the Torah. To be accepted into the priesthood strict measures were in order, since the Jews believed that true worship can only be conducted and led by properly qualified men. Only the descendants of the sons of Aaron could be priests, although the descendants of Levi can perform a limited number of functions.
At the head of the priesthood and head of the Jewish people was the High Priest who was also a political leader who negotiated with neighboring governments. From the beginning of Hebrew history the High Priest sprinkled the blood of sacrifice on the mercy seat at Yom Kippur and led the Jewish people in Temple worship. It is important to note that the high priest was different than the chief priests who were the heads of priestly families.