Blockchain basics: blocks, hashes and consensus
A plain-language walkthrough of what a blockchain actually is and why it is hard to rewrite.
A blockchain is a list of records, called blocks, that are linked together. Each block contains a cryptographic fingerprint, or hash, of the block before it. Changing an old block would change its hash and break every link after it.
Many independent computers keep a copy of the chain. They follow shared rules, called a consensus mechanism, to agree on which new block comes next. Proof of work and proof of stake are the two best-known approaches.
This structure makes the history very hard to alter quietly, which is why blockchains are used to record ownership of digital assets. It does not make every application built on a blockchain secure: smart contracts and websites can still contain bugs.
If you are new to the topic, start by learning how a wallet, a transaction and a block explorer fit together. Seeing your own test transaction appear on a public explorer makes the ideas concrete.