A fork occurs when two miners find a valid hash within a short space of time. They both spread the solution for verification to their neighbors. As the process continues the network will split into two, where one half of the network believes one block is the next to be included in the blockchain ledger, whilst the other half of the network believes in a different block.
Click the link below for a better explanation of a bitcoin fork by Nolan Bauerle, the director of research at CoinDesk.
http://www.businessinsider.com/bitcoin-fork-explained-gold-segwit-segwit2x-cash-the-bit-3-2017-10
thanks for sharing
He excellently explained it. Thank you Apple :)