What is the difference between USDT and USDT ERC20?

Could you elaborate on the key distinctions between USDT and USDT ERC20? I'm particularly interested in understanding the technical and functional differences between these two cryptocurrencies. For instance, does the ERC20 designation suggest a specific type of blockchain technology or platform they operate on? Also, how do these differences affect their usability, transactions, and overall value within the cryptocurrency ecosystem? It would be helpful if you could provide a comparative analysis of their features and functionalities, highlighting the unique characteristics of each. Thank you for your insights.