Tether is a tocken issued by Tether Limited company which belongs to the Bitfinex trading platform. It was built on top of Omni Layer digital currency protocol, which works on the Bitcoin blockchain.
The main purpose of this token is to provide stability to traders and members of cryptocurrency market, because Tether is claimed to be backed by US dollars on Tether Ltd bank accounts, so its price is not as volatile as prices of other cryptocurrencies. However, that remains yet to be proven and is is the reason why the U.S. Commodity Futures Trading Commission issued subpoenas to both Tether and Bitfinex. Also Tether Ltd underlines that the Tether token is not a financial instrument, and the owners of Tethers have no rights for legal claims or guarantees against losses.
Nevertheless, tether is a very popular token and is used by a number of exchanges (mostly the crypto to crypto only ones) as their main stablecoin. The amount of Tethers grew up to $2.8 billion. During summer 2018 around 80% of Bitcoin volume was traded in Tethers.