What is UST vs USDT?

Could you please explain the key differences between UST and USDT? I'm curious to know what sets them apart from each other in the world of cryptocurrencies. Are they both stablecoins? If so, how do they differ in terms of their underlying assets, issuance mechanisms, and adoption rates? Additionally, are there any notable risks associated with each of them that investors should be aware of? Thank you for your insights.