Fork
In short: A fork is a blockchain split into two versions. It happens when part of the network starts following different rules, or a different chain of blocks…
A fork is a blockchain split into two versions. It happens when part of the network starts following different rules, or a different chain of blocks, than the rest; the word is also used when developers copy an open-source project's code to start a separate one.
Why it matters
Forks are how blockchains change their rules, and they come in two main types. A soft fork tightens the rules in a backward-compatible way, so nodes that do not upgrade can still follow the chain. A hard fork changes the rules in a way older software rejects; if part of the community refuses the change, two separate chains and coins can result. Around a planned fork, exchanges often pause deposits and withdrawals of the affected coin until the network is stable.
Example
Bitcoin Cash was created through a hard fork of Bitcoin after a long dispute over block size. Anyone who controlled bitcoin at the block where the chains split ended up with the same amount of Bitcoin Cash on the new chain. From that block on, the two networks have kept separate histories, rules and markets.