Are USD and USDT the same?

Excuse me, I've been hearing a lot about cryptocurrencies recently and I'm trying to wrap my head around the differences between them. Could you clarify something for me? Are USD and USDT the same thing? I've seen them mentioned interchangeably in some places, but I'm not quite sure if they're the exact same thing or if there's some crucial distinction I'm missing. It would really help me if you could break it down for me in a simple way. Thank you in advance for your time and assistance.