Is a USD and a USDT the same?

I'm somewhat confused about the difference between a USD and a USDT. Are they interchangeable, or are there significant distinctions? As I understand, USD stands for the United States Dollar, the official currency of the United States. But what exactly is USDT, and how does it differ from the traditional dollar? Is it a digital representation of the USD, or does it have its own unique characteristics and uses? I'm trying to wrap my head around the nuances of cryptocurrency and its relationship to traditional fiat currencies, and this is one of the areas where I feel a bit hazy. Could you please clarify the difference between these two terms for me?