USDT, or Tether, is a stablecoin that was created in 2014 by a company called Tether Limited. The company's goal was to create a digital currency that was pegged to the value of the US dollar, which would allow users to conduct transactions without being subject to the volatility of other cryptocurrencies.

Initially, USDT was built on the Bitcoin blockchain using the Omni protocol. However, in 2017, Tether Limited started to migrate USDT to other blockchains, including Ethereum and Tron.

USDT operates by holding reserves of US dollars and issuing tokens that are backed by those reserves. Each USDT token is supposed to be worth one US dollar, and Tether Limited claims that it holds enough reserves to back all of the USDT in circulation.

However, USDT has been the subject of controversy and criticism. Some have raised concerns about the company's claims of having enough reserves to back all of the USDT in circulation, and there have been questions about the company's financial transparency. Additionally, some have accused Tether Limited of artificially inflating the price of Bitcoin by using USDT to buy large amounts of the cryptocurrency.

Despite these concerns, USDT has remained a popular stablecoin and is widely used in cryptocurrency trading. As of 2021, there are billions of USDT tokens in circulation, and the stablecoin is supported by many cryptocurrency exchanges and wallets.