A blockchain node is a computer that stores a copy of the blockchain and helps validate new transactions and blocks against the network's rules. Nodes are what make a blockchain genuinely decentralized rather than dependent on any single server, a concept central to how a blockchain works. Both major networks publish guidance on running one: Bitcoin.org and the Ethereum Foundation.
Node types
Full nodes store the entire blockchain history and independently verify every rule. Lighter nodes store less data and rely partly on full nodes for verification, trading some independence for lower resource requirements.
The distinction people most often blur is between a node and a miner or validator, which are not the same job. A full node verifies and relays; it enforces the rules but never proposes a block, and it earns nothing for the work. A miner or validator additionally competes to add new blocks and gets paid for it. Every miner runs a node, but the overwhelming majority of nodes do no mining at all. They are there to check the work of the ones that do.
Light clients make the opposite trade. Rather than storing hundreds of gigabytes, a light client keeps only block headers and asks full nodes for anything else it needs. That is what lets a wallet run on a phone, and it is a sensible compromise for most people, but it does mean taking someone else's word for the parts of the ledger it never downloaded. Archive nodes sit at the far end: they retain every historical state the chain has ever had, which almost nobody needs except explorers and analytics services.
Validation
Nodes check that new transactions and blocks follow the network's rules, correct signatures, no double-spending, valid format, before accepting them. This distributed checking is what prevents any single participant from cheating unnoticed.
The consequence is that miners have far less power than their share of the network suggests, and this is the point most explanations skip. A miner controlling most of the hashpower could reorder recent transactions or censor them, which is bad enough. What that miner cannot do is pay themselves coins that do not exist, spend from a wallet they lack the key to, or raise the supply cap. A block containing any of those is rejected by every full node on the network, including nodes belonging to other miners, and the work spent producing it is simply wasted.
That is the substance behind the phrase "don't trust, verify." A full node is not a formality or a redundancy. It is the thing that makes the rules binding, because a rule that nobody independently checks is a rule the checker can change.
Decentralization
The more independent nodes a network has, spread across different operators and locations, the harder it becomes for any single party to control or corrupt the ledger. Node count and distribution are often cited as a rough proxy for a network's decentralization. Our walkthrough of blocks, nodes and consensus shows how this ties together.
Count alone is a crude measure, though, and worth reading skeptically. Ten thousand nodes all hosted in the same data center, run by the same handful of operators, or dependent on the same cloud provider are not ten thousand independent checks. What matters is how many genuinely separate parties could refuse a rule change, and whether it would be practical to pressure or unplug them all at once. Distribution across jurisdictions, hosting providers, and client software all feed into that, and none of it shows up in a headline node count.
This is also why the hardware requirements for running a node are a live political question rather than a technical footnote. Every increase in the storage, bandwidth, or memory needed to participate prices some operators out, and the ones who drop off are always the hobbyists rather than the institutions. A chain that can only be verified by companies with server racks has, in practice, handed rule enforcement to those companies, whatever its architecture diagram says.
Running a node
Anyone can typically run a full node on a public blockchain, given enough storage and bandwidth, participating directly in verifying the network rather than just trusting others to do so.
In practice it is less dramatic than it sounds. A Bitcoin full node needs a few hundred gigabytes of disk, an always-on connection, and unattended patience during the initial sync, which takes anywhere from hours to a couple of days as it replays and verifies the entire history from the first block. Ethereum asks for more, and a fast SSD is not optional there. Neither requires special hardware, and plenty of people run one on a spare mini PC in a cupboard.
What you get in return is specific rather than financial: nothing to earn, but your wallet stops asking a third party what your balance is and checks the chain itself. Nobody can serve you a filtered view of history, and your privacy improves, since you are no longer telling someone else's server which addresses you care about. For most holders that is more assurance than they need. For anyone whose stake is large enough that they would rather verify than trust, it is the only way to actually do so.
There is a collective argument for it too, beyond your own balance. Every additional independent node makes the network marginally harder to pressure, because rule changes need the cooperation of the people running the software, and a rule the node operators reject does not take effect no matter who proposes it. Nodes are cheap to run and impossible to buy out wholesale, which is precisely why a network with many of them, held by many unrelated people, is worth more than one with a few large ones.