Tether was designed to solve the problem for which cryptocurrency world is associated with, which is volatility. Formerly realcoin but rebranded in 2014 and started trading in 2015, tether was a product of Tether Ltd a company incorporated in British Virgin Island.
Tether company is to staked 1 tether coin to 1 usd dollar and it also promise to keep in bank 1 dollar for every tether coin in circulation. Bitfinex exchange shares common owners and managers with tether. Some people have accused Tether of double dealing because they don't do according to their promise, because as they promised the account where the equivalent dollars of tether coin is kept will be audited regularly and made public, but since it's inception nothing as such has happened.
So though, tether coin is popular and has saved many from losing their money, it has also helped in faster transfer of fund and is almost in any exchange it still remain unregulated, should it become valueless all the holders will lose their investment with no where to turn to for insurance.
Tether coin is available is ethereum based, trx based and omni based.