What is the difference between ERC-20 and USDT?

I'm curious about the differences between ERC-20 and USDT. Could you please elaborate on this? ERC-20, I understand, is a technical standard used for smart contracts on the Ethereum blockchain, allowing for the issuance of fungible tokens. But USDT, I'm not as familiar with. Is it a specific type of ERC-20 token? Or does it operate differently? What are the main characteristics that distinguish these two? And how do investors typically use them in their portfolios? I'm keen to understand the nuances and how they fit into the broader cryptocurrency and finance landscape.