A blockchain is a database that no single person controls. Copies exist on thousands of computers around the world. When a new transaction is broadcast, nodes check it against the rules and relay it to their peers. Miners or validators package valid transactions into blocks. Each block links to the one before it through a cryptographic hash. Change a past block and every hash after it breaks.

That structure makes records tamper-evident. It does not make them true. A blockchain records what the network agrees happened. If someone tricks the network into accepting a false transaction, the ledger reflects that falsehood. The security comes from the cost of rewriting history, not from magic. Bitcoin's proof-of-work makes rewriting expensive. Proof-of-stake chains use economic penalties instead of electricity.

Core properties

• Distributed — many copies, no single point of failure
• Append-only — new data added, old data hard to change
• Transparent — anyone can audit the ledger
• Consensus-driven — nodes agree on the valid state

Public blockchains are open to anyone. Permissioned blockchains restrict who can read or write. Most enterprise blockchain projects use the permissioned model. Most cryptocurrency networks are public.