What Is a Smart Contract? How They Work, With Examples

Summary: A smart contract is a program stored on a blockchain that runs automatically when its conditions are met. Nobody has to approve each step, and once deployed the code usually cannot be changed. Smart contracts power token swaps, lending apps, NFTs and much of what people call DeFi. They remove the middleman, but they also remove the safety net, so a bug or a careless approval can be expensive.

• A smart contract is code, not a legal document, and it lives on a blockchain such as Ethereum.
• It executes the same way for everyone, with no company in the middle deciding.
• Most contracts cannot be edited after launch, which is both the strength and the risk.
• The biggest danger for ordinary users is signing an approval they do not understand.

What Is a Smart Contract?

A smart contract is a piece of software that lives at an address on a blockchain. It holds rules written in code, and it carries them out whenever someone sends it a transaction that meets those rules. If the conditions are met, the contract acts. If they are not, it refuses. There is no customer service desk and no manual override.

The idea is older than crypto. Computer scientist Nick Szabo described smart contracts in the 1990s, using the vending machine as his example: put in the right amount, choose an item, and the machine delivers it without a shopkeeper. Blockchains made the idea practical at scale, because they give the code a shared, tamper-resistant place to run.

How Smart Contracts Work

On Ethereum, the best-known smart contract platform, developers usually write contracts in a language called Solidity and deploy them to the network. From then on the contract has its own address, just like a wallet, and it can hold funds and data.

• A user sends a transaction to the contract, for example asking it to swap one token for another.
• Every node on the network runs the same code and checks that the result is valid.
• If the rules are satisfied, the contract updates balances or data on the blockchain.
• The user pays a network fee for the computation, which on Ethereum is known as gas.

That fee is why busy periods get expensive: every contract call competes for space in the next block. The mechanics are covered in more detail in the guide to what a gas fee is and why it changes.

Smart Contract Examples You Have Probably Seen

Most people use smart contracts without thinking of them that way. Common examples include:

• Token swaps: decentralized exchanges use contracts that hold pools of tokens and set prices automatically.
• Lending and borrowing: contracts hold deposits, calculate interest and handle collateral without a bank.
• Stablecoins and tokens: most tokens on Ethereum, including many stablecoins, are themselves smart contracts that track who owns what.
• NFTs: an NFT is a record inside a contract that says which wallet owns a specific item.
• Multi-signature wallets: contracts that only release funds when several people sign, often used by teams and organizations.

Types of Smart Contracts

There is no single official list, but contracts tend to fall into a few broad groups. Some simply hold and move value, such as escrow or payment splitting. Some manage assets, such as tokens and NFTs. Others run whole applications, such as exchanges, lending markets and on-chain games, often by linking many contracts together.

Not every blockchain handles them the same way. Bitcoin supports only a limited scripting language on purpose, which keeps it simpler and harder to break. Ethereum and many newer networks, such as Solana, were built to run far more complex programs.

Smart Contract Security: Where Things Go Wrong

Because contracts run automatically and usually cannot be edited, mistakes stick. One of the most famous examples is The DAO in 2016, where a flaw in the code let an attacker drain a large share of the funds it held. Projects now pay for audits, but an audit lowers the risk rather than removing it.

For everyday users, the more common problem is not a broken contract but a malicious one. Scam sites ask you to connect a wallet and approve a transaction that quietly gives a contract permission to move your tokens. A few habits cut that risk a lot:

• Read what a transaction is asking for before you sign it, especially unlimited token approvals.
• Only connect your wallet to sites you reached yourself, not through ads or direct messages.
• Keep long-term holdings on a hardware wallet such as Ledger, which shows transaction details on its own screen before you confirm.
• Avoid signing anything on public Wi-Fi, or use a VPN such as NordVPN if you have to.

What to Check Before You Sign

Smart contracts are one of the most useful ideas in crypto: they let strangers trade, lend and build without trusting a company in the middle. The trade-off is that you become your own safety check. Understand what a contract will do, sign only what you meant to sign, and keep your keys offline where you can. If self-custody is the next step, browse Trezor gift cards.