hard fork, soft fork
3.1 Example | BCH <-> BTC
On August 1st, 2017, the original bitcoin chain split into 2 separate blockchains. Bitcoin (BTC), and Bitcoin Cash (BCH). Since this change introduced incompatibles, miners had to choose which fork to support. Each gained enough support to continue, and live on as independent project with their own communities, developers, and miners.
After the fork, for every bitcoin on the pre-fork blockchain, there now existed 1 Bitcoin Cash (BCH) and 1 Bitcoin (BTC)
3.2 Example | BTC <-> BTG
Bitcoin Gold (BTG) was another fork of the bitcoin chain, but it failed to gain enough community and miner support to make an impact, and soon became irrelevant
3.3 Example | BCH schedule
Bitcoin Cash (BCH) undergoes regularly planned hard-forks to update the network, currently scheduled every 6 months.