Aaron was Moses's older brother. He was born in 2365 (of the Jewish calendar), three years before Moses, before the Pharaoh's edict requiring the death of male Hebrew children. He was the ancestor of all koheins, the founder of the priesthood, and the first Kohein Gadol (High Priest). Aaron and his descendants tended the altar and offered sacrifices. Aaron's role, unlike Moses's, was inherited; his sons continued the priesthood after him (Num. 20:26).
Aaron served as Moses's spokesman. As discussed above, Moses was not eloquent and had a speech impediment, so Aaron spoke for him (Ex. 4:10-16). Contrary to popular belief, it was Aaron, not Moses, who cast down the staff that became a snake before Pharaoh (Ex. 7:10-12). It was Aaron, not Moses, who held out his staff to trigger the first three plagues against Egypt (Ex. 7:19-20; Ex. 8:1-2 or 8:5-6; Ex. 8:12-13 or 8:16-17). According to Jewish tradition, it was also Aaron who performed the signs for the elders before they went to Pharaoh (Ex. 4:30).
Aaron's most notable personal quality is that he was a peacemaker. His love of peace is proverbial; Rabbi Hillel said, "Be disciples of Aaron, loving peace and pursuing peace, loving people and drawing them near the Torah." According to tradition, when Aaron heard that two people were arguing, he would go to each of them and tell them how much the other regretted his actions, until the two people agreed to face each other as friends.
In fact, Aaron loved peace so much that he participated in the incident of the Golden Calf (Ex. 32), constructing the idol in order to prevent dissension among the people. Aaron intended to buy time until Moses returned from Mount Sinai (he was late, and the people were worried), to discourage the people by asking them to give up their precious jewelry in order to make the idol, and to teach them the error of their ways in time (Ex. 32:22).
Aaron, like Moses, died in the desert shortly before the people entered the Promised Land (Num. 20). [He was 123 years old.]
Sources: Judaism 101