What is a blockchain, and what problem did Bitcoin first solve with it?
A blockchain is a shared, append-only ledger replicated across many independent computers; Bitcoin used it to let strangers transact peer-to-peer without any bank or central authority in the middle.
Before Bitcoin, sending value online always required a trusted intermediary (a bank, a payment processor) to keep the books and prevent you from spending the same money twice. The pseudonymous author "Satoshi Nakamoto" published the Bitcoin white paper describing how a network of participants could instead agree on one shared history of transactions using cryptography and economic incentives — no central operator, and no single point that can be censored or shut down. Because the supply is fixed and the ledger is hard to rewrite, many people treat Bitcoin as a scarce "digital gold" store of value. The deeper breakthrough, though, is the coordination mechanism itself: a way for mutually distrusting parties to agree on a single truth. That mechanism is what everything else in this topic builds on.
Go deeper:
Learn Blockchain, Solidity & Full Stack Web3 — freeCodeCamp — the 32-hour Patrick Collins course; its opening lesson walks these foundations end to end.
Bitcoin: A Peer-to-Peer Electronic Cash System (Satoshi Nakamoto) — the original nine-page white paper that first solved double-spending without a bank.- Blockchain — Wikipedia — broad overview of the shared-ledger data structure and its history.