Hard Fork
A permanent, backward-incompatible change to a blockchain's protocol rules that requires all nodes to upgrade to the new software version. Nodes that don't upgrade continue following the old rules, effectively creating two separate chains. Hard forks can be planned upgrades or contentious splits when the community disagrees on the direction.
“Ethereum Classic (ETC) exists because of a hard fork after the 2016 DAO hack — the community split over whether to reverse the theft, creating two separate chains: ETH (reversed) and ETC (unchanged).”
Soft Fork
A backward-compatible change to a blockchain's protocol rules where upgraded nodes enforce new rules while older nodes still recognize the new blocks as valid. Soft forks don't split the blockchain because old nodes can still participate, though they don't enforce the new rules. Soft forks are considered less disruptive than hard forks.
Node
A computer that maintains a copy of the blockchain and participates in validating and relaying transactions. Full nodes store and verify the complete blockchain history independently. Light nodes store only block headers for efficiency. Archive nodes retain every historical state of the blockchain. The more nodes a network has, the more decentralized and resilient it becomes.
Consensus Mechanism
The method by which a decentralized blockchain network reaches agreement on the current state of the ledger and which transactions are valid. Consensus mechanisms solve the problem of coordinating untrusting parties without a central authority. Major types include Proof of Work, Proof of Stake, Delegated Proof of Stake, and Proof of Authority.
Blockchain
A distributed, append-only digital ledger that records transactions across a network of computers in a way that makes it cryptographically secured and practically impossible to alter historical records. Each block contains transaction data and is linked to the previous block through a cryptographic hash, forming an immutable chain.