Are a USD and a USDT the same?

I'm a bit puzzled about the difference between a USD and a USDT. Could you please clarify for me? Are they interchangeable, or do they serve different purposes in the realm of cryptocurrency and finance? I've heard that USDT is somehow pegged to the value of the US dollar, but I'm not entirely sure how it differs from a regular USD. Could you elaborate on the distinctions between these two, and maybe explain their respective roles in the crypto ecosystem? Your expert insight would be greatly appreciated.