A node is a computer connected to a blockchain network that maintains a copy of the ledger and participates in validating and relaying transactions according to the network rules.
Types of nodes: Full nodes store complete blockchain history, Light nodes store only block headers, Archive nodes store all historical states, and Validator nodes participate in consensus.
Why run a node: Verify transactions independently, support network decentralization, access blockchain data directly, and contribute to network security.
Node requirements vary by network: Bitcoin requires 500GB+ storage, Ethereum requires 1TB+ (archive node much larger), and some networks have lower requirements.