Soft fork: tighter rules
A soft fork makes the rules stricter: some blocks that were valid become invalid, but every block valid under the new rules is still valid under the old ones. Nodes that do not upgrade keep following the chain. SegWit and Taproot were soft forks.
Hard fork: looser rules
A hard fork accepts blocks the old rules reject, for example bigger blocks. Nodes that do not upgrade refuse them. If both sides persist, the chain splits into two coins with a shared history up to the split.
- Soft fork: old nodes still follow along; requires broad support to activate safely.
- Hard fork: everyone must upgrade or be left on another chain.
- Bitcoin has preferred soft forks for its upgrades.