A blockchain fork occurs when a blockchain diverges into two separate paths, usually as a result of a disagreement among developers or a major change in the underlying protocol. Forks can be categorized as soft forks or hard forks. A soft fork is backward-compatible, meaning that nodes running the old version of the software will still recognize new blocks as valid. A hard fork, on the other hand, is not backward-compatible, leading to a permanent split in the blockchain. Notable examples include Bitcoin Cash (BCH), which forked from Bitcoin, and Ethereum Classic (ETC), which forked from Ethereum.