Uniswap

Uniswap is a decentralised exchange protocol on Ethereum and other EVM chains. Instead of an order book, trades execute against liquidity pools that anyone can deposit into, with prices determined by a formula rather than by matching a buyer to a seller. The design is what made it influential. A token can be traded as soon as someone creates a pool for it, without a listing process, and the pool's liquidity providers earn a share of the trading fee. The interface is a website, but the exchange itself is a set of contracts, so anyone can build another interface or route trades through it. The tradeoffs are inherent to the model. A large trade moves the price against itself, which is why aggregators split orders across pools. Liquidity providers face loss relative to simply holding the assets when prices diverge, and the user experience requires a wallet, gas, and an understanding of what a failed transaction costs.