Dai (DAI) is a stablecoin - a cryptocurrency which price is linked to an asset - that is pegged to the US Dollar (USD) through stabilizing mechanisms built into smart contracts on the Ethereum blockchain.
Linked to the Maker (MKR) system, which is built on the Ethereum network, the price of 1 DAI is maintained to be equivalent to 1 USD. Whenever there is a deviation of price of 1 DAI from the value of 1 USD, the Ethereum-based smart contract would work to bring the price back to exactly 1 USD, either through the creation or deduction of MKR tokens. DAI can be freely traded like any other ERC20 token, is fully decentralized and anyone with an ETH wallet can own and manage it autonomously.