Uniswap is currently the largest decentralized exchange (DEX) by trading volume operating on the Ethereum blockchain, which allows anyone to swap ERC20 tokens.
dYdX does have a larger trading volume than Uniswap, but it uses an order book rather than liquidity pools.
UNI token is Uniswap’s governance token, that provides Uniswap users the chance to get involved in the project’s governance.
Uniswap was the pioneer in using the automated market maker model, where users supply Ethereum tokens to liquidity pools, and algorithms determine how to price assets.
The users that supply the tokens to the various pools earn rewards for doing so.
As of July 2022, Uniswap has Total Value Locked (TVL) of USD $6.05 billion and upwards of USD $2 billion in trading volume.