Tether is something of a hybrid. It’s a crypto-fiat “stablecoin.” Its value is pegged or rather, tethered to the value of fiat currencies.
USD₮ is the currency that Tether most frequently uses. More specifically, 1 USD₮ is intended to remain exactly equal to $1, not a cent more or less. It’s basically a crypto dollar (at least in value).
Another currency that Tether uses is the Euro, in the form of EUR₮. And Tether will reportedly use the Japanese Yen soon, in the form of JPY₮.
Technically, Tether is based on the Omni Platform. This platform is used for various digital assets, which are anchored to the Bitcoin.
To sum up, the only reason why 1 USD₮ is actually worth $1 is that the exchanges keep a reserve of dollars to back every USD₮ in existence.