Blockchain

A blockchain is a shared ledger that thousands of independent computers keep identical copies of, with new entries bundled into blocks and chained to the block before them. Once a block is buried under enough later blocks, rewriting it would mean out-computing the whole network, which is what makes the history practically tamper-proof.

The point is that no single company runs the ledger. When you send Bitcoin, you are not asking a bank to update its private database; you are broadcasting a transaction that the network validates and records, and anyone can read the result. That openness is the feature and the burden: every balance is public, and there is no help desk to reverse a mistake.