Is USD and USDT the same?

I'm a bit confused here. Could you clarify for me if USD and USDT are indeed the same? I've heard some discussions about them, but I'm still not entirely sure. USD, as I understand, is the abbreviation for the United States Dollar, the official currency of the United States. But what exactly is USDT? Is it some sort of digital representation of the US Dollar? Or is it a completely different entity with its own unique characteristics? I'd really appreciate it if you could break it down for me in a way that's easy to understand. Thank you in advance for your help!