What is the difference between Sharia and religion, and what does the dominance of Islam over all previous messages mean?

There is no difference between Sharia (definition) and Din, as each term is applied to the other. Sharia may be used to mean established practices and rulings, as in the Almighty's saying: "To each of you We prescribed a law and a method." Din may be used to denote monotheism, as in the Prophet's (peace be upon him) saying: "We, the community of prophets, are brothers from different mothers; our religion is one." Linguistically, Sharia is the custom and the way, and the water source into which animals enter, or what Allah has legislated for His servants concerning religion. Din is that to which a person submits and obeys. The meaning of Islam's dominance over other messages is its preservation of them, its testimony over them, and its abrogation of them.