Is USDT the same as Ethereum?

Is USDT truly identical to Ethereum? Could you clarify the difference between these two cryptocurrencies for me? I've heard they're both used in blockchain technology, but I'm not sure if they serve the same purpose. USDT seems to be a stablecoin, while Ethereum is a smart contract platform. Could you elaborate on their functionalities and how they differ from each other? Is it possible to interchangeably use them in transactions, or do they have distinct roles in the crypto ecosystem? I'm keen to understand the nuances between these two cryptocurrencies.