A timeline of all the major milestones, forks, and updates to the Ethereum blockchain. What are forks?
Changes to the rules of the Ethereum protocol which often include planned technical upgrades. More Forks are when major technical upgrades or changes need to be made to the network — they typically stem from Ethereum Improvement Proposals EIPs and change the btc fork of the protocol.
When upgrades are needed in traditional, centrally-controlled software, the company will just publish a new version for the end-user. Blockchains work differently because there is no central ownership.
Ethereum clients must update their software to implement the new fork rules. Plus block btc fork miners in a proof-of-work world, validators in a proof-of-stake world and nodes must create blocks and validate against the new rules.
More on consensus mechanisms These rule changes may create a temporary split in the network. New blocks could be produced according to the new rules or the old ones.
Forks are usually agreed upon ahead of time so that clients adopt the changes in unison and the fork with the upgrades becomes the main chain. However, in rare cases, disagreements over forks can cause the network to permanently split — most notably the creation of Ethereum Classic with the DAO fork.