A blockchain is a type of distributed ledger: a shared record of transactions maintained collectively by a network of computers, rather than by a single bank, company or government. Once entries are recorded and confirmed, they are extremely difficult to alter.
Blocks, chains and consensus
Transactions are grouped into blocks. Each block contains a cryptographic fingerprint (hash) of the previous block, linking them into a chain. To add a new block, network participants must agree it is valid — a process called consensus. Because every copy of the ledger can verify the chain's history, tampering with an old entry would require rewriting every block after it on a majority of copies simultaneously.
Public vs private blockchains
Public (permissionless) blockchains let anyone join and validate, prioritizing openness and censorship resistance — at the cost of speed and energy use, depending on the consensus design. Private or permissioned blockchains restrict participation to known organizations, trading openness for efficiency and control; they are essentially shared databases with cryptographic audit trails.
Real uses beyond speculation
The best-known application is cryptocurrency, but the ledger concept is also used for supply-chain tracking, digital identity pilots, land registries in some jurisdictions, and smart contracts — self-executing agreements whose terms are written in code. Many proposed uses remain experimental, and conventional databases are simpler and faster when a trusted central party is acceptable.
Honest limitations
Blockchains do not magically guarantee truth: if bad data is entered, the ledger faithfully preserves bad data ("garbage in, garbage out"). They can be slow and energy-intensive compared with centralized systems, key loss can mean irreversible loss of assets, and the technology has attracted significant fraud — healthy scepticism is warranted.