A blockchain is a decentralized, distributed ledger that records transactions across many computers so that the registered transactions cannot be altered retroactively. It serves as the underlying technology for various digital innovations, including cryptocurrencies like Bitcoin and Ethereum. On the other hand, a smart contract is a self-executing contract with the terms of the agreement directly written into lines of code. It lives on the blockchain and automatically executes transactions once certain pre-set conditions are met. So, while blockchain is the underlying technology providing the environment and security for various digital interactions, smart contracts are one of the applications that operate on top of the blockchain, utilizing its decentralization and security features to execute automated, trustless transactions.