What Is a ‘Hard Fork’ versus a ‘Soft Fork’ in Blockchain Technology?

A 'hard fork' is a permanent, non-backward-compatible divergence in the blockchain protocol. It requires all nodes to upgrade to the new rules; if they don't, they operate on a separate, incompatible chain.

A 'soft fork' is a temporary, backward-compatible change where non-upgraded nodes will still see new blocks as valid, but they may not enforce the new, stricter rules. Soft forks are generally less disruptive.