A blockchain is a distributed ledger — a database replicated across thousands of independent computers (nodes) worldwide.

Blocks and hashes: Each block contains a batch of transactions plus a cryptographic hash of the previous block. Because every block references the one before it, changing any historical record would invalidate every subsequent hash — making the chain immutable.

Example: Block #800,000 contains: - Transactions (Alice Bob: 0.5 BTC) - Timestamp - Previous block hash: `0000a3f7...` - Own hash: `0000d9c2...`

Why it matters for investors: Immutability removes the need for a central authority (bank, clearinghouse). Transactions settle peer-to-peer, typically in minutes rather than days (T+2 in traditional finance).

Key properties: - Decentralized: no single point of failure or control - Transparent: all transactions are publicly auditable - Permissionless: anyone can participate without approval