Is BNB chain and BNB smart chain the same?

Excuse me, I'm trying to understand the difference between BNB chain and BNB smart chain. Are they essentially the same thing, or do they have distinct functionalities and purposes? I've heard them mentioned interchangeably, but I'm not quite sure if they are interchangeable in practice. Could you clarify this for me, please? It would be greatly appreciated if you could provide a concise explanation of their similarities and differences, if any, in a way that's easy to understand for a non-expert like me.