Explaining Forks

A block chain is a history of all transactions. New transactions are grouped into a block and appended to the chain. This appending of blocks is done separately by all miners, so there is a possibility that two miners may append blocks to their copies of the chain at the same time. When other miners see these two new versions of the chain, they must pick one to append their blocks to. Eventually one will be longer than the other just by random chance, and at that point, all miners will switch to that chain because that is the rule.

Now, miners only accept valid chains. A fork occurs when some nodes believe that one version of the chain is valid but the rest of the nodes believe that is is not valid. If the two groups of nodes cannot agree, then there is a permanent fork as each group continues to append blocks to their version of the chain.

This can be done on purpose, as in the case of Bitcoin Cash. At the announced time, a Bitcoin Cash miner added a block to the chain that Bitcoin Core nodes considered to be invalid. Bitcoin Cash nodes consider the chain with this block to be valid so they continue with the version of the chain that includes the block. Bitcoin Core nodes consider the block to be invalid, so they continue with a version of the chain without the block. Since they can't agree, there is a permanent fork in the chain at that block.