It's a centralized cryptocurrency that creates tokens at will.

Here are some good starting points for learning more about Tether:

https://medium.com/@bitfinexed/the-mystery-of-the-bitfinex-tether-bank-and-why-this-is-suspicious-a8a6407a1241

https://cointelegraph.com/news/tether-really-isnt-a-scam-company-promises

https://cointelegraph.com/news/the-strange-story-of-tether-the-digital-money-that-claims-it-isnt-money

https://www.coindesk.com/tether-claims-30-million-stable-token-stolen-attacker/