When nodes disagree on the canonical chain, a fork occurs. Forks are resolved by the longest-chain rule (actually the most cumulative proof-of-work). Two types exist:

Soft fork — backward-compatible rule tightening; old nodes still accept new blocks
Hard fork — backward-incompatible rule change; old nodes reject new blocks, creating two chains

Historical examples:

Fork	Type	Year	Outcome
BIP 66 (signature validation)	Soft	2015	Unified chain
SegWit (BIP 141)	Soft	2017	Unified chain
Bitcoin Cash	Hard	2017	Chain split
Ethereum Merge	Hard (consensus)	2022	Unified chain

Key insight: Soft forks require only a majority of miners to activate, while hard forks require all nodes to upgrade or the network splits.