The Bitcoin software has been released on January 2009 by Satoshi Nakamoto.
The Bitcoin software is open source. Means the software code is openly available. Everyone can change the code to implement different features.
A hard fork of a blockchain will happen, if some blockchain nodes run an altered software, validating different consensus rules. If one group of nodes continues to use the traditional software while the other nodes use altered software, a split will occur.
There are several hard forks of the Bitcoin blockchain:
- Bitcoin Cash (BCH), hard forked July 2017
- Bitcoin Gold (BTG), hard forked in October 2017
- Bitcoin Diamond (BCD), hard forked in November 2017
- Bitcoin SV (BSV) is a hard fork from Bitcoin Cash from November 2018.
Beware! There are more cryptocurrencies called Bitcoin.
To name a few: Lightning Bitcoin, Bitcoin Platinum, Super Bitcoin, Bitcoin God, etc.
Bitcoin Core (BTC) is the original Bitcoin.