A blockchain may be defined as a distributed ledger of transactions. Like a traditional ledger, individual transactions (unique blocks) are added to the ledger (the chain) and never removed.
If you have access to the latest block, it is possible to access all previous blocks linked together in the chain. A blockchain database retains the complete history of all assets and instructions executed since the very first one — making its data verifiable and independently auditable.
Trust in the ledger comes from the process itself rather than from the status of any one participant. In this secure, shared database, participants have their own copies of the stored data.
Strong cryptography provides that transactions can be initiated only by certiﬁed parties, that changes are validated by participants collectively, and that the outputs of the system are immediate, accurate and irrevocable. Distributed ledgers are inherently harder to attack, as a cyber attack would have to attack all the copies simultaneously to be successful.
Therefore, a complete audit trail is maintained throughout the chain by the distributed nature of the information. Anyone with the appropriate encryption rights can access a copy of that ledger and verify past transactions without having to trust the participants in the original transaction.