The Maker token (MKR) is the MakerDAO native token. It acts as a utility token, governance token and a recapitalization tool for the MakerDAO platform, which manages the DAI stablecoin. MKR is used as part of the stabilizing mechanism that is in place to combat the price volatility of the DAI stablecoin.
DAI is pegged to the US dollar and has collateral in place to ensure stability. The MKR token is used is to pay for the "collateralized debt positions" (CDPs) that have been used to generate DAI. After this MKR is used, it is removed from the supply. MKR holders also contribute to the governance of the MakerDAO and vote on crucial issues related to the direction of the DAI stablecoin.

USD Coin is a stablecoin created by Coinbase and Circle. It's backed by USD (1 USDC=$1 USD), and is designed to be a stable digital currency that will fit within the trust and regulatory frameworks of the financial system, while still benefiting from the transparency and advantages of crypto powered networks. USDC is issued using the CENTRE open source framework and membership scheme, and it is available as a framework that multiple companies can leverage. Eventually other financial institutions will be able to become USDC issuers (and issuers of other fiat stablecoins - e.g. EURC, JPYC, GBPC), and customers will be able to tokenize USD and redeem USDC tokens in an open and interoperable ecosystem of financial institutions. A membership governance model for issuers will ensure that they meet technical, operational, regulatory, compliance and audit requirements.

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.