A chain split happens when nodes follow different rules, so some accept a block that others reject. Each group keeps building on its own chain.
Accidental splits
In March 2013 nodes running 0.8 accepted block 225,430 while many older nodes rejected it because of a database lock limit (BIP 50). Miners downgraded so the network reunited on one chain within hours.
Deliberate splits
A contentious hard fork creates a lasting split: Bitcoin Cash diverged after block 478,558 on 1 Aug 2017. Both chains share history up to the split, so anyone holding coins before it holds coins on both.
Why it matters
Every user who runs a full node decides which rules they accept. That is why changing Bitcoin is slow – and why it is hard to capture.