Yield farming is the practice of earning rewards by depositing cryptocurrency into DeFi protocols. Users provide liquidity (ETH/USDC to Uniswap) or lend assets (USDC to AAVE), earning trading fees + farming rewards (protocol tokens). Farmers chase high APY by moving capital between pools—a constant optimization game. Example: Yearn Finance automates this: you deposit USDC → Yearn moves it between AAVE, Curve, Compound chasing best yield → you earn compounded returns.