A hard fork in blockchain technology refers to a radical change to a blockchain network’s protocols. Hard forks divide a single network into two and cause previously-valid transactions to be considered invalid. Valid blocks generated utilising the new rules may be considered as invalid, or invalid blocks may be perceived as valid, necessitating the upgrade of all nodes intended to work with the new rules.
Following the addition of the new rule, one path will follow the new blockchain while the other will continue to follow the old one. A lasting divide can arise if one group of users uses the old software while the others use the new software. Such was the case with Bitcoin Cash, which split off from the main Bitcoin blockchain in a contentious event back in 2017.
In some cases, a hard fork may not be contentious at all. Rather, a blockchain may implement a hard fork as a way to upgrade the network, in which case users using the old chain realise their version is outdated and less useful than the new one, and choose to upgrade to the new one.