Smart contracts

A smart contract is a program that lives at an address on a blockchain and runs deterministically when called, with its state and code visible to everyone. "Smart" is a misnomer — they're not AI, they're just code with no off-switch. The actual shift they represent: instead of a company promising to hold your money and do the right thing, the rules are written in public code that anyone can audit and nobody (not even the deployer, usually) can unilaterally change. That's the whole basis for DeFi, DAOs, NFTs, and almost everything else on Ethereum. The trade-off is brutal: bugs are forever, exploits cost real money, and "we'll fix it in v2" means redeploying and migrating everyone.