A hard fork is the division of a single blockchain into two divergent chains, in which the rules of the newly created chain are incompatible with those governing the original chain. That means nodes looking to participate in the validation of blocks on the new network must upgrade their software so that it aligns with the new rules.
Original network participants who don’t perform the upgrade won’t be able to validate on the updated blockchain, resulting in two separate blockchains existing at the same time, each with its own rules. Transaction histories will also be kept separate from the moment the hard fork occurred.
Hard forks are often introduced to fix existing protocol problems, improve features, or add functionalities. Sometimes a hard fork will be formed as a result of disagreements over governance, with one part of the community adopting the original chain, while another segment may decide to take on a new direction, as was the case with the Bitcoin Cash hard fork in 2017.
