What is a node?

A node is software that connects to other nodes, downloads blocks and transactions, and checks them against Bitcoin’s rules. A full node checks every block from the genesis block on and keeps the current set of unspent outputs. It does not take anyone’s word for what is valid.

Rules and preferences

Consensus rules are the ones every node must agree on: the block limit, the subsidy schedule, valid signatures, no coin spent twice. A block that breaks one is rejected, however much work it carries.

Policy is each node’s own choice about unconfirmed transactions: minimum fees, dust limits, replacement rules. Policy can differ between nodes and versions. That is one reason mempools are similar but rarely identical, and why a transaction one node refuses can still be valid in a block.

Miners propose, nodes check

Miners decide which transactions go into a block, and in what order. They do not decide the rules. A block that pays its miner more subsidy than the schedule allows is rejected by full nodes, and the work spent on it is wasted. That is how the halving and the supply cap are enforced: by every node checking, not by any company announcing it.

Running your own

A wallet has to ask some node about the chain. If it asks a company’s server, you trust that server’s answers and you tell it which addresses are yours. Connecting your wallet to your own node removes both. A node needs a computer that stays on, a steady internet connection, and disk space for the chain, or for a pruned copy that keeps only recent blocks.

Running a node is a separate choice from holding your own keys. You can do one without the other. Doing both is the stricter setup.