What Is a Hard Fork versus a Soft Fork in Cryptocurrency?

A hard fork is a permanent divergence from the previous version of the blockchain that is not backward-compatible. Nodes must upgrade to the new rules, or they will remain on the old chain.

A soft fork is a temporary divergence that is backward-compatible. Nodes that do not upgrade will still recognize new blocks as valid, but they will not be able to enforce the new, stricter rules.

Hard forks are more likely to create a replay attack vector.