A blockchain fork occurs when the blockchain's protocol undergoes a change, creating a divergence in the chain’s transaction history or rules. Forks can happen for various reasons, including upgrades, disagreements within the community, or the need to address security vulnerabilities.

There are two primary types of blockchain forks: hard forks and soft forks.

Hard forks involve a permanent split from the existing blockchain. This occurs when a new set of rules is implemented that is not compatible with the older version of the blockchain. Nodes that do not update to the new protocol cannot validate transactions on the updated chain. This can result in two separate blockchains and, in many cases, two different cryptocurrencies. Notable examples include Bitcoin Cash (a hard fork of Bitcoin) and Ethereum Classic (a hard fork of Ethereum).

Soft forks, on the other hand, are backwards-compatible updates. They introduce changes to the blockchain’s protocol without requiring all participants to upgrade. Non-upgraded nodes can still validate transactions, but they may not support newer features.

Forks play a crucial role in blockchain development, enabling the network to evolve, fix bugs, and meet changing user or developer demands. However, they can also lead to community divisions and technical challenges.