What Is a Blockchain?

How a Blockchain Stores and Protects Data

Each block in a blockchain contains three essential components: a batch of transaction data, a timestamp, and a cryptographic hash of the preceding block. That hash — a fixed-length fingerprint computed from the previous block's entire contents — is what creates the chain. Change a single character anywhere in the historical record and its hash changes, which breaks the link to the next block, which breaks the link to the one after that, and so on through every subsequent block. An attacker would need to redo the computational work for every modified block and all that follow before the rest of the network notices and rejects the altered chain.

The ledger is distributed, meaning thousands of independent computers (nodes) each maintain a full or partial copy. There is no single server that can be taken offline to erase or rewrite history. When a new block is proposed, nodes check it against the protocol rules and the existing chain; they only accept blocks that are valid. This consensus process — the mechanism by which all nodes agree on a single authoritative version — is what distinguishes different blockchains: Bitcoin uses proof of work, Ethereum uses proof of stake, and other networks use variations like delegated proof of stake or proof of authority.

A common misconception is that "blockchain" and "cryptocurrency" are synonymous. Blockchains are the underlying data structure; cryptocurrencies are one application built on top of them. The same technology is being explored for supply-chain provenance records, digital identity systems, voting mechanisms, and real estate title registries — any use case where an auditable, tamper-evident record shared among parties who don't fully trust each other is valuable.

Key Points

• Blocks are linked by cryptographic hashes — each block commits to the entire history before it, making retroactive edits computationally prohibitive.
• Decentralization removes single points of failure. No one entity controls the canonical record; consensus rules enforced by independent nodes determine which chain is valid.
• Public blockchains are transparent by default — every transaction is visible to anyone running a node or using a block explorer.
• Not all blockchains are equal in decentralization, security, or throughput. Tradeoffs between these properties — often called the blockchain trilemma — vary across different networks.

Learn More

The blockchain is the foundation that makes Bitcoin, Ethereum, and the broader crypto ecosystem possible. See Crypto Fundamentals for a complete walkthrough of how cryptocurrencies are built on top of blockchains and how to evaluate projects that use them.

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Related Questions

What is the difference between a public and private blockchain?

A public blockchain (like Bitcoin or Ethereum) is permissionless — anyone can read, write, or participate in validation without approval. A private blockchain restricts participation to authorized parties, typically used by enterprises for internal record-keeping. Public blockchains offer stronger censorship resistance and transparency; private blockchains offer faster throughput and access control but sacrifice the trustless, open-verification properties that make public blockchains valuable.

How does a blockchain confirm transactions?

When a transaction is broadcast to the network, nodes independently verify it against the blockchain's rules — checking digital signatures, ensuring funds haven't already been spent, and validating format. Valid transactions wait in a mempool until a block producer bundles them into a block and adds it to the chain. The more blocks added on top, the more computationally expensive it becomes to reverse, which is why multiple confirmations are considered final.