Smart contract
In short A smart contract is a program stored on a blockchain that runs exactly as written when called, holding funds and enforcing rules without a middleman.
A smart contract is code deployed to a blockchain address. Once it is live, anyone can send it a transaction, and every node runs the same code to agree on the result. Tokens, DEX pools, lending markets, NFT collections, and bridges are all built from smart contracts.
Mechanically, a contract has functions and stored state, such as a list of balances. Calling a function that changes state costs a gas fee, which pays validators to execute and record it. On EVM chains like Ethereum, BNB Chain, and Base, contracts are usually written in Solidity and compiled to bytecode. Solana calls them programs, and token balances there live in separate accounts managed by a shared token program rather than one contract per token.
For a token buyer, the contract is the real rulebook. Whatever the website or the chat says, the code decides whether the owner can mint new supply, charge a tax on sells, pause trading, or blacklist wallets. Transactions sent to a contract cannot be undone, so a bug or a hostile rule hits everyone holding the token at once.
Before buying, check that the source code is verified on a block explorer so you can read what it does, not just trust bytecode. Look for owner-only functions and whether ownership was renounced or handed to a multisig. Watch for proxy contracts, since an upgradeable contract can be swapped for different code later. An audit helps, but it only covers the code that was reviewed at the time it was reviewed.