A blockchain is a digital record of transactions. The name comes from its structure, in which individual records, called blocks, are linked together in single list, called a chain.
Blockchains are used for recording transactions made with cryptocurrencies, such as Bitcoin, and have many other applications.
Each transaction added to a blockchain is validated by multiple computers on the internet. These systems, which are configured to monitor specific types of blockchain transactions, form a peer-to-peer network.
They work together to ensure each transaction is valid before it is added to the blockchain. This decentralized network of computers ensures a single system cannot add invalid blocks to the chain.
When a new block is added to a blockchain, it is linked to the previous block using a cryptographic hash generated from the contents of the previous block. This ensures the chain is never broken and that each block is permanently recorded. It is also intentionally difficult to alter past transactions in blockchain since all the subsequent blocks must be altered first.