Blockchain is a distributed ledger technology that maintains a growing list of records, called blocks, which are securely linked together using cryptography. Each block contains a cryptographic hash of the previous block, transaction data, and a timestamp. This design makes it inherently resistant to data modification: once recorded, the data in any given block cannot be altered retroactively without altering all subsequent blocks, which requires network consensus.
Blockchains can be public (open and decentralized, like Bitcoin), private (controlled by a single entity), or consortium-based (managed by a group of organizations). They underpin various digital currencies but are also used for a wide array of applications such as smart contracts, supply chain management, voting systems, and identity verification, offering enhanced security, transparency, and reduced risk of fraud compared to traditional database systems.