A hard fork is a major protocol upgrade that forces all users to switch to new software if they want to continue using the same software. chain of blocks.
Unlike a Soft Fork, in a hard fork there is a change in the code of a cryptocurrency which makes the new version incompatible with the previous versions. This process that causes what we call 'network fork'; you have to choose between using the new version of the software or the previous one.
The reasons why it occurs may be either "accidental" due to a correction of an error in the code. Or premeditated creating a new protocol proposal and applying them in a new version of code.
To better understand what a Hard Fork is, let's imagine a trip with friends through the forest. As long as you continue together, following the same path, you will all see the same. But if at a certain moment a part of the group decides to follow a different route, the experience will not be the same. When a hard fork occurs, the past knowledge of the path is shared but not the knowledge of future paths.
A hard fork can originate two types of chains (the original and the new one) giving rise to two different networks and currencies. As it has happened in Bitcoin several times. One of the best known hard forks Bitcoin es Bitcoin Cash.
Bitcoin Cash is a sought after fork of Bitcoin with the aim of improving some features. It inherits almost all the Bitcoin code (they use SHA-256, both generate blocks every 10 minutes,…). The differences are that Bitcoin has 1MB blocks y supports 3.5 transactions per second among other things, while in Bitcoin Cash the blocks are up to 32MB and supports about 24 transactions per second.