Tether (USDT) is the largest stablecoin cryptocurrency. It is pegged to the US Dollar. (1 USDT=$1 USD) Each Tether token that is issued is backed by $1 USD that is supposedly held in reserve. Tether has been the subject of some controversy because they have failed to allow a transparent audit of their currency reserves.
Tether facilitate transactions between cryptocurrency exchanges at a rate that is fixed to the United States dollar. This allows traders to take better advantage of arbitrage opportunities. Tether was created by the leaders of Bitfinex, one of the largest crypto exchanges.

HUSD token is a stablecoin that was created by a Hong Kong based company called Stable Universal. HUSD is backed by US Dollars in a 1:1 ratio that are held in a trust company. The HUSD stablecoin is designed to have a governance structure that will increase security, liquidity, and transparency. Monthly audits of HUSD's reserves are conducted by an independent accounting firm to ensure that the 1:1 backing is maintained.

TrueUSD is a stablecoin backed by USD. It was created by TrustToken, a company that specializes in creating asset-backed tokens on the Ethereum blockchain. TrueUSD is fully collateralized and is transparently verified by third-party attestations.
TrueUSD was created within a legal framework that enables users to exchange USD directly with an escrow account. The platform is powered by smart contracts that ensure a 1:1 parity between USD and TrueUSD. Escrow accounts and regulatory compliance practices enable auditing and more legal protection for token holders.
Users wire funds to these third-party escrow accounts, and an equivalent amount of TUSD is generated to their ethereum addresses. Each TUSD can be redeemed for one US Dollar, that is held in escrow.