What is the difference between ERC20 and USDT?

Could you please elaborate on the fundamental differences between ERC20 and USDT? I'm particularly interested in understanding their technical distinctions, as well as their practical applications within the cryptocurrency ecosystem. ERC20, as I understand, is a token standard on the Ethereum blockchain, but how does it vary from USDT, which is a stablecoin designed to maintain a pegged value? Additionally, could you explain the impact of these differences on traders, investors, and the broader cryptocurrency market? Thank you for your insights.