1. Hard Fork: Say a group of nodes go rogue, or has a potential disagreement with the blockchain (or with something that is being introduced to the original blockchain). Now, the said group will decide to go their own way eventually leading to a “forked” version of this blockchain. Rest of the nodes will now have the option to update their systems in order to work with this blockchain or stick to the original one. The important thing here is that the nodes running with previous blockchain are not allowed in the new one. A hard fork essentially leads to creation of two separate blockchains.
Probably the best explanation of Hard Fork I’ve ever read.
Thanks for sharing.