Uniswap is a decentralized exchange that uses liquidity pools (LPs) to make markets instead of a typical order book. These pools are defined by smart contracts that facilitate the swapping of tokens and the adding of liquidity — there are no order books, centralized parties, or central facilitators. Uniswap users pool two assets that are then traded against, with the price determined by the ratio between the two. Anyone with ERC-20 tokens can add liquidity to these pools by adding an equal value of each token to the LP. When providing liquidity to an LP, users receive a LP token that is redeemable for the underlying assets plus fees at any time, and fees are evenly distributed amongst the individual pool.
UNI is the governance token native to Uniswap.