Is Ethereum the same as USDT?

Excuse me, I'm new to the world of cryptocurrencies. I've heard a lot about Ethereum and USDT, but I'm not sure if they're the same thing. Could you please clarify the difference between the two? I understand that Ethereum is a blockchain platform, but what is USDT and how does it relate to Ethereum? Are they interchangeable or do they serve different purposes? Thank you for your help in clearing up this confusion.