Aaron was Moses' prophet, and Moses was God's prophet. The prophet is a message carrier from one of greater authority. In this case, Moses was in the position of God to Aaron, as well as to Pharaoh. By combining Exodus 4:10-16 with Exodus 7:1-2, the biblical usage of a prophet has a good foundation. A prophet is one who expresses the will of God in words and sometimes with signs given to confirm what is said.
Through Moses, the function of a prophet begins to be established: to cry aloud and show men their sins (Isaiah 58:1). It does not stop there, though, because they were also pastors and ministerial monitors of the peoples' conduct and attitudes. Their function differed from that of priests in that the priest approached God by means of sacrifice on behalf of the people. The prophets, by contrast, approached men as ambassadors of God, beseeching them to turn from their evil ways and live (Ezekiel 33:11).
The difference between a prophet and a priest is a matter of direction, in that one goes from God to man (the prophet), and the other goes from men to God (the priest). It is also a matter of directness. The priest is indirect, while the prophet is direct. We have things going in opposite directions here, yet both working to accomplish essentially the same thing, which is to bring man and God into a relationship with one another. This has direct application to us under the New Covenant (II Corinthians 5:20-21).