A blockchain is a distributed ledger — a database copied across thousands of computers (nodes) worldwide. Every node holds an identical copy.

Each block contains: - A batch of validated transactions - A timestamp - A cryptographic hash of the previous block

That last point is the key insight: each block is mathematically linked to the one before it. To alter a past transaction you would have to recompute that block's hash *and* every block after it, on more than half of all nodes simultaneously. This makes tampering computationally infeasible.

Blockchains eliminate the need for a central authority (bank, clearinghouse) to verify and record transfers. Trust is enforced by math and distributed consensus instead.