What is the difference between BNB and BUSD?

I'm curious about the distinction between BNB and BUSD. Could you please elaborate on their fundamental differences? For instance, I'd like to know how they differ in terms of their functionality, usage scenarios, and their roles within the cryptocurrency ecosystem. Additionally, it would be helpful if you could compare their market performance and popularity among investors. As a professional in the field of cryptocurrency and finance, your insights would greatly assist me in understanding these two tokens better.