A blockchain fork occurs when the existing code base of a blockchain is modified, resulting in the potential creation of two blockchains operating under different set rules. There are two main types of forks: soft forks and hard forks. A soft fork is a backward-compatible upgrade that allows non-upgraded nodes to function normally, while a hard fork results in a permanent divergence from the original blockchain, meaning that nodes that do not upgrade will no longer be able to validate transactions on the new chain. Forks can occur for various reasons, including implementing new features, fixing bugs, or responding to community disputes.