What is a Fork?
A change to Bitcoin's protocol rules. A soft fork is backward-compatible. A hard fork is not and creates a permanent split, resulting in two separate cryptocurrencies (like Bitcoin and Bitcoin Cash in 2017).
Why It Matters
Forks are how Bitcoin evolves, but they're also points of disagreement. Soft forks are upgrades that tighten rules—they're backward-compatible because stricter validation still accepts blocks that the old rules would accept. Hard forks are more radical changes that expand what's allowed, making old software unable to validate new blocks. Most forks are soft forks that the network gradually adopts through consensus. Hard forks are contentious because they require universal agreement; if one group doesn't upgrade, you end up with two separate cryptocurrencies. Bitcoin Cash was created through a hard fork by miners and developers who disagreed with Bitcoin's direction. Understanding forks helps you realize that Bitcoin's direction depends on community consensus, not central authority.
How It Works
A soft fork is like adding a new traffic rule that's stricter than before. Old software ignores the new requirement, but new software enforces it. Nodes running old software still see new blocks as valid. SegWit was a soft fork that changed how transaction data is stored; old nodes treated it as valid though they didn't fully understand the new format. A hard fork is like changing which side of the road people drive on. Old software and new software become completely incompatible. If 90% of miners upgrade to a hard fork and 10% don't, the 10% continues mining on the old rules, creating a split blockchain. Both versions use the name "Bitcoin," but they're separate cryptocurrencies from the hard fork forward.