In plain English
What Proof of work means
A proof-of-work system sets a difficulty target for block hashes. Miners repeatedly vary data and hash it until one result meets that target. Other participants can verify a successful result quickly. Because changing an old block would require redoing its work and the work of later blocks, accumulated work helps make confirmed transaction history costly to rewrite.
Why it matters
Proof of work underpins the security model of networks such as Bitcoin, but it does not guarantee that an individual payment is final or that an asset will retain value. Security depends on factors including the distribution of mining power, economic incentives, software rules, and the amount of work accumulated after a transaction’s block.
Example
Suppose a miner assembles a candidate block and must find a hash below the network’s current target. There is no shortcut: it may need to try a very large number of different inputs before finding one valid hash. Once found, other nodes can check that hash quickly. Altering one transaction would change the block hash and require new proof of work.
Quick answers
Common questions
Why does proof of work use energy?+
Miners perform repeated computations while competing to find a hash meeting the protocol’s difficulty target. The required computational effort is the resource that makes rewriting past blocks expensive relative to honestly extending the chain.
Does the longest blockchain always win in proof of work?+
More precisely, proof-of-work networks generally select the valid chain with the greatest cumulative work, subject to their consensus rules. Length by block count alone can be an incomplete description because blocks can represent different amounts of difficulty.
Sources