A blockchain is a decentralized, digital ledger that records transactions across a network of computers. Each block in the chain contains a number of transactions, and every time a new transaction is added to the block, it is added to the chain, creating a permanent and unchangeable record of all the transactions in the block. This technology is most commonly associated with Bitcoin and other cryptocurrencies, but it can also be used for other purposes such as supply chain management, voting systems, and more