A smart contract is code that runs on a blockchain. It executes automatically when conditions are met. No lawyer, no notary, no escrow agent. The code is the agreement. If you send 1 ETH to a contract that promises to send back a token, the contract either does it or the transaction fails. There is no middle ground and no one to appeal to. That finality is the point. It is also the risk. Bugs in smart contracts have lost hundreds of millions of dollars. The DAO hack in 2016 drained $60 million. The Parity wallet freeze locked up $150 million. Code is law, and law can have bugs.

Ethereum introduced smart contracts in 2015. Solidity is the most common language. Developers write the contract, test it on a testnet, audit it, and deploy it. Once deployed, the contract is immutable unless it includes an upgrade mechanism. That upgrade mechanism is a centralization risk. If one wallet can change the contract, the contract is not truly decentralized. Most major DeFi protocols use multisig wallets or governance votes to control upgrades. The trade-off between immutability and upgradeability is one of the central tensions in smart contract design.

Smart contract essentials

• Self-executing — runs automatically when conditions are met
• Immutable — cannot be changed after deployment unless designed to be
• Transparent — code is public on the blockchain
• Auditable — anyone can review the logic

Read the contract before you interact with it. If you cannot read the code, find someone who can. Or stick to protocols that have been audited and battle-tested.