When a blockchain separates into two branches, this is known as a fork. Up until the break, these two branches shared a common transaction history. After that, they all move forward on their own, independently. They can be divided into accidental and intentional.
Sometimes, thousands of miners are vying with one another to build a new block. Due to the volume of mining activity, it is occasionally possible for two or more miners to create a new block. This can result in an unintentional fork. New blocks are added to one of the chains, which solves the issue. The network does not reconverge on a single chain after an intentional fork. The blockchain community uses this kind of fork to update the protocol.