A hard fork splits a blockchain into two but allows both branches to keep working simultaneously. Creating this permanent divergence can help upgrade the blockchain as long as the community all follow the new fork and the old fork is left behind. In some rare cases, the community itself may be divided about which fork they support and both forks may continue. Ethereum implemented a hard fork to reverse the hack on the DAO, resulting in a permanent split between Ethereum and Ethereum Classic.
“Bitcoin Cash is a hard fork on the Bitcoin blockchain that went about as well as a shit sandwich.”« Dictionary Menu