What is the difference between a denarius and an aureus?

Could you please elaborate on the distinct differences between a denarius and an aureus? I'm particularly interested in understanding their historical significance, monetary value, and the materials they were typically made from. Was there a specific period of time where one was more prevalent than the other, and if so, what factors contributed to this? Furthermore, were they both used interchangeably as a form of currency, or did they serve different purposes? Thank you for your insights.