Node
A node is a computer that keeps a copy of the blockchain and checks every transaction and block against the network's rules. It is nodes, not miners, that decide which rules count as valid.
How it works
A full node downloads the network's entire history and verifies it itself, from the first block to the latest. It trusts nobody: every signature, every amount and every rule is checked by its own code.
A block that breaks the rules is simply rejected, even if the largest miner produced it. Which is why rules cannot be changed against the nodes' will: the network accepts only what they consider valid.
There are also light nodes that store no full chain and query full ones for data. A phone wallet works that way: fast, at the cost of trusting whoever answers.
Why run your own
- Verification without a middlemanYour own node shows the network's state by its own rules rather than on some other server's word.
- PrivacyA wallet querying someone else's node tells that node which addresses interest you.
- A voice in rule disputesIn a fork, nodes choose which version of the rules to enforce. Without one you accept someone else's choice.
- Requirements are lower than expectedA full bitcoin node runs on an ordinary mini-PC with a couple of terabytes of disk.