What's the difference between USD and USDT?

Could you please explain the difference between USD and USDT to me? I've been hearing about these two terms a lot in the cryptocurrency and finance world, but I'm still a bit confused about their distinctions. USD, I understand, is the abbreviation for the United States dollar, which is a fiat currency widely used in global transactions. But what exactly is USDT? Is it a type of cryptocurrency? And how does it differ from USD in terms of functionality, value, and usage? I'd really appreciate it if you could clarify these points for me.