Blockchain is a distributed digital ledger technology that records transactions across a network of computers in a secure, transparent, and tamper-resistant way. Each record, called a block, contains a list of transactions and is linked cryptographically to the previous block, forming a continuous chain. This structure ensures that data cannot be altered retroactively without altering all subsequent blocks and obtaining consensus from the network participants, making blockchain useful for applications like cryptocurrencies, supply chain management, and digital identity verification.