Aaron (meaning "mountaineer" or "rich, fluent") was the brother of Moses and speaker to the Pharaoh on behalf of his brother. Most significantly, Aaron would be the first High Priest and would be the first to Preside over God's priesthood and Tabernacle. While the Levitical Priesthood was made up of the Tribe of Levi, only actual descendants of Aaron would serve in actual priestly positions.
Aaron was born to the slaves Amram and Jochebed, both descendants of Levi; in Goshen, Egypt while Israel was enslaved. At the time he had an older sister named Miriam. When he was three years old he had a younger brother born to him named Moses. However; due to a decree from the Pharaoh demanding that all Hebrew male infants were to be killed, Moses was not around for Aaron's life. By miraculous circumstance Moses was adopted into the Royal Family of Egypt and lived far away from the place where the Israelites lived.
Aaron lived his life as a slave growing up with his older sister Miriam under his parents Amram and Jochebed. In time Aaron was married to a Judahite named Elisheba, the daughter of Amminadab and sister of Nahshon (future Chief of Judah). Aaron had four sons with his wife Elisheba: his firstborn, Nadab followed by Abihu, Eleazar and Ithamar.
Aaron to speak for MosesWhen Aaron was around eighty-three years old, Moses returned to Egypt from his exile in Midian in order to free Israel from the oppressive regime. While Moses and Aaron had not grown up together they recognized each other, as God had instructed each of them individually to work together. Aaron was told to go into the wilderness to meet up with his brother, so he traveled up to Mount Sinai where Moses was journeying to Egypt. He greeted his brother with a kiss and Moses explained all the signs God had performed through him.
Moses was to be a powerful figure, like a god before Pharaoh. Since Moses had fear in speaking before Egypt's King, Aaron who happened to be a great speaker, would speak on Moses behalf. As an analogy, Aaron was to be a " prophet" for Moses,just as God instructed.
Aaron extends his staffAfter this Aaron and Moses gathered the family heads of the Israelites. Moses gave Aaron the words to speak and his prophet he spoke the words to the Israelites, while Moses performed signs. The Israelites were astonished to see this and knowing that God sought their freedom they bowed and worshipped.
Aaron and Moses went before the King of Egypt and demanded that Israel be allowed to go into the wilderness to worship God. The Pharaoh, enraged at their request to give the slaves rest decided to punish the slaves and made the work more difficult. Incidentally the Israelites reacted harshly to Moses and Aaron.
Once again Aaron went before the King of Egypt, speaking on behalf of his brother. The Ruler asked for proof with a miracle, so Aaron took his staff and when he threw it down it transformed into a snake. The Egyptians repeated this miracle using their own craft and their staffs transformed into snakes, however they were devoured by Aaron's snake-staff.
God had already told the brothers that the Pharoah's heart would be hardened and he would not succumb to their requests, so he would cast plagues on Egypt. While the Pharaoh was in the Nile River bathing, Aaron extended his staff at the instruction of Moses. All the water in Egypt turned to blood by the hand of the God of Israel, manifested in Aaron's staff. 
Aaron did this twice more, inflicting plagues of frogs and gnats. The King of Egypt still did not allow Israel its freedom to worship God, so several more plagues occurred; this time with Moses stretching out his hand rather than Aaron. Aaron continued to accompany Moses in his pleads before the Pharaoh, oftentimes speaking on his behalf. Many times Aaron was given revelation concerning future plagues alongside Moses.
Free from EgyptOnly after all of the firstborn Egyptians were killed during the Passover would Egypt become so annoyed with the Israelites that they would set them free. During the Exodus, Aaron continued to share a joint-office along with Moses. They would both receive word from God and the two of them would be responsible for implementing it, often in different capacities. There were few instances were Aaron had a task that was not delegated to both him and Moses.
In the wilderness the Israelites began to regret leaving Egypt, because they no longer had bountiful amounts of food. Moses and Aaron promised the people they would receive from God. Aaron proclaimed that God had heard the Israelites complaints and immediately a cloud rose in the sky. Later that evening the the first flock of quail came and filled the Israelite camp, giving bountiful meat. In the morning a grinded bread, called Manna was found all over the ground at the camp. After a week of Manna Moses instructed Aaron at God's word to take an omer of the Manna and place it in a jar, as memorandum of the Manna for generations. Aaron dedicated the jar of bread to God, designating a foreshadow to his future position as Priest.
Battle with the AmalekitesWhen Israel camped at Rephidim, near Mount Sinai the Amalekites attacked the Israelites. Moses saw the Amalekites army approaching so the next day Joshua lead the Israelite army in battle. Aaron viewed the battle from a nearby hill, alongside Moses and Hur, probably because they could not fight since they were Levites. From the hill Moses would hold his staff in the air, which give the Israelites prosperity in battle. Moses eventually became tired, so Aaron with Hur had to help prop Moses arms in the air to continue to give the Israelites prosperity in battle.
Giving of the Law Dining with JethroAfter the battle with the Amalekites, Jethro, the father-in-law of Moses came to visit. During this time Moses summoned the tribal elders of Israel to come and dine with him. Aaron came alongside the elders of Israel.
Revelations of GodSometime later, when Moses had returned from a trip on Mount Sinai God commanded him to bring Aaron along with him. When Moses returned to Sinai Aaron came along with him. Therefore, Aaron was present for the revelation of the Ten Commandments and the very first laws of Israel.
After that episode of revelation had ended everyone returned down to Israel's camp, until Moses was to be re-summoned to Sinai. When Moses was preparing to convene with God on Sinai, Aaron was called along with his two eldest sons and seventy elders of Israel. God gave some of his presence to Aaron, his sons and the assembly; and they feasted while beholding God. Moses was called back on the mountain in order to receive more laws concerning the future of Israel, in particular Aaron and his sons. Before Moses left, Aaron was set in charge (alongside Hur) to handle any disputes that arose to a national level.
During Moses communion with God a large set of rules were established that Aaron would be personally responsible for implementing.
The Golden CalfSince Moses was on Mount Sinai for a prolonged period of time, the Israelites began to worry. With Aaron left in charge of any court cases or disputes, the people naturally went to him for a solution. Aaron may have been resting or inactive from his duties in the interim, unless the need arose. The people came to Aaron asking him to arise and to make idols for their protection. In response Aaron told the people to bring all of their gold items to him. After having the gold smelted, Aaron carved an image of a golden calf; perhaps to resemble the gods of Egypt. When the people saw this idol as a symbol of the pantheon that "lead them out of Egypt", Aaron built an altar and declared the following day to be a special feast of God. Aaron did not keep a tight rein on the people as he was supposed to, so they became involved in all sorts of evil.
When Moses returned to see the golden calf he was completely decimated. Rhetorically, Aaron was asked by Moses what the people had done to convince him to lead them into sin. Aaron asked Moses to not become foul tempered, and blamed the Israelites and offered an explanation of what he did.Afterwards the people experienced a plague, as a result of Aaron's making of a calf.
Another time after Moses returned from Sinai, his face was shining so much from being in God's presence Aaron was afraid.
High Priest of Israel
OrdinationBy Yahweh's command Aaron was to be officially ordained as Priest of Israel. Aaron was washed by his brother in water and was given his official garments created for his new status. Oil was poured onto Aaron's head as a mark of his appointment by Deity. Afterwards, Aaron was instructed to wait at the entrance of the Tabernacle for seven days and nights.
On the eighth day, Aaron made several sacrifices for the atonement of his own sin, followed by all the sacrifices that the Israelites gave.