A node is a computer/server running blockchain network software. Nodes verify transactions and blocks and keep a copy of the ledger. A network can have several types of nodes, such as full nodes, validators, miners, archive nodes, and light clients.
Having lots of nodes helps decentralization, but not all nodes carry equal power. Validators/miners taking part in consensus are the ones mainly responsible for producing blocks.
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Nodes are basically the network's inspectors and bookkeepers rolled into one.