A blockchain is a distributed ledger with growing lists of records (blocks) that are securely linked together via cryptographic hashes. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data (generally represented as a Merkle tree, where data nodes are represented by leaves). Since each block contains information about the previous block, they effectively form a chain (compare linked list data structure), with each additional block linking to the ones before it. Consequently, blockchain transactions are irreversible in that, once they are recorded, the data in any given block cannot be altered retroactively without altering all subsequent blocks. (資料來源 )
By storing data across its peer-to-peer network, the blockchain eliminates some risks that come with data being held centrally.[3] The decentralized blockchain may use ad hoc message passing and distributed networking. In a so-called “51% attack” a central entity gains control of more than half of a network and can then manipulate that specific blockchain record at will, allowing double-spending. (資料來源 )