Nodes, validators and their roles
Who holds a copy of the ledger, who proposes blocks, and how the two groups overlap.
A node is any computer running network software that connects to peers and handles the ledger. Full nodes download blocks, replay every transaction and reject anything that breaks the rules, which means they trust no one else's word. Light nodes keep only block headers and ask full nodes for the details they need, trading some independence for much lighter hardware. Archive nodes go further and store the complete record of past states, which suits researchers and services that query history. Software that runs a node is called a client, and many networks support several independent clients so that one coding mistake does not take every node down together.
Producing blocks is a different role. A miner on a proof-of-work network or a validator under proof of stake assembles a block and proposes it, but the block still has to pass inspection by the full nodes around it. A node can refuse a block even when a validator signed it, which is why the number and spread of independent nodes matter. If only a handful of operators ran nodes, they could agree on rules that suited themselves. Many separate operators in many places make a quiet rule change much harder to push through, and make outages easier to survive.
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