What is a hard fork?
A change that loosens Bitcoin's rules, so nodes running old software reject the new blocks. Without near-universal agreement it splits the chain.
A soft forkSoft forkA change that tightens Bitcoin's rules, so nodes that haven't upgraded still accept the new blocks. tightens the rules and old nodes still accept the blocks. A hard fork goes the other way — bigger blocks or new rules old software calls invalid — so everyone must upgrade before the switch, or the network splits in two.
Splits have happened on purpose: Bitcoin Cash broke away in 2017 over block size, and the market then priced the two coins separately. A hard fork is also the name for the moment two coins share one history up to the split.
References
- Wikipedia, “Bitcoin Cash”.