A cryptocurrency fork is a split in the blockchain network that occurs when a cryptocurrency's existing code is modified, resulting in two separate versions of the blockchain. This can happen for various reasons, such as disagreements among developers, changes in protocol rules, or the need to fix bugs or improve the system. There are two main types of forks: soft forks, which are backward-compatible and do not create new coins, and hard forks, which are not compatible with previous versions and result in the creation of a new cryptocurrency.