How blockchains work: blocks, hashes and nodes

A blockchain is a ledger, a list of records, that is copied across many computers called nodes. Transactions are grouped into blocks, and each block contains a fingerprint called a hash that depends on the block's contents and on the hash of the block before it. Because each block refers to its predecessor, changing an old record would change every later hash, and the other nodes would notice the mismatch. This linking is the basic reason blockchain records are described as tamper-evident. The design is meant to make changes to past records detectable rather than impossible.

The hard part is agreeing on which blocks are valid and in which order. Networks use a consensus mechanism for this. In proof of work, participants compete to solve a difficult puzzle, which takes energy and time. In proof of stake, participants lock up a digital asset as a deposit and are chosen to propose or check blocks, and they can lose that deposit if they cheat. Every design has limits: throughput is finite, confirmation takes time, and a record is only as trustworthy as the software and the people who run it. Check these limits before you rely on any network for an important record.